Impl ToString for Builder, accept AsRef<[Node]> in `render()`

This adds a to_string method to the `Builder` allowing for the easy conversion of a Vec<Node> into a String, for any usecases where a library might not be directly writing to an io::Write, or may want to do String-y things with your document first.  Without this, users would have to write to a Vec<u8> and convert to a String, which is kinda unintuitive, takes a lot of steps, and doesn't produce very readable code.  This simplifies it to one method call.

* Implementation of the std::str::ToString method for Builder
* Accepting any AsRef<[Node]> in render (including accepting the old Vec<Node>, so not breaking)
* Addition of estimate_len() to Node, used to pre-allocate the correct size of the String buffer

* `estimate_len` has some quick doctests and examples.  I know most of the rest of the project uses test methods, but I hope this is alright given that the tests may add some more clarity to the purpose and function of the method.
* `to_string` has a single line of unsafe code.  As the associated comment explains, this is provably safe, and exists just to avoid having to choose between having a bunch of duplicate code or inefficiently performing a UTF-8 check on a whole bunch bytes that we already know are safe.  That said, I totally get it if you're just generally against unsafe code and will change it to be an alternative if you so wish
* ToString is implemented instead of Display.  This is to discourage users from directly using this in a println!() or write!() macro, which would not be a thing you would normally expect to do with this.  It also gives us the advantage of being able to pre-allocate a buffer size, meaning less expensive String resizing.
* I couldn't think of a clever way to get `render()` to work with both `io::Write`s or `fmt::Write`s without duplicating the code, but I'm dumb and might be missing something, so if there's a way to do that instead of doing my funky unsafe hack that's cool and I can do that instead.

gemtext/src/lib.rs

@@ -54,11 +54,30 @@ impl Builder {
     }
 }
 
+impl ToString for Builder {
+    /// Render a document to a string
+    ///
+    /// This produces a text/gemini compliant text document, represented as a string
+    fn to_string(&self) -> String {
+        let len: usize = self.nodes.iter().map(Node::estimate_len).sum(); // sum up node lengths
+        let mut bytes = Vec::with_capacity(len + self.nodes.len()); // add in inter-node newlines
+        render(&self.nodes, &mut bytes).unwrap(); // Writing to a string shouldn't produce errors
+
+        unsafe {
+            // This is safe because bytes is composed of Strings.  We could have this as
+            // pure safe code by replicating the `render()` method and switching it to use
+            // a fmt::Write (or even `String::push()`)instead of a io::Write, but this has
+            // the same effect, with much DRYer code.
+            String::from_utf8_unchecked(bytes)
+        }
+    }
+}
+
 /// Render a set of nodes as a document to a writer.
-pub fn render(nodes: Vec<Node>, out: &mut impl Write) -> io::Result<()> {
+pub fn render(nodes: impl AsRef<[Node]>, out: &mut impl Write) -> io::Result<()> {
     use Node::*;
 
-    for node in nodes {
+    for node in nodes.as_ref() {
         match node {
             Text(body) => {
                 let special_prefixes = ["=>", "```", "#", "*", ">"];
@@ -72,7 +91,7 @@ pub fn render(nodes: Vec<Node>, out: &mut impl Write) -> io::Result<()> {
                 None => write!(out, "=> {}\n", to)?,
             },
             Preformatted(body) => write!(out, "```\n{}\n```\n", body)?,
-            Heading { level, body } => write!(out, "{} {}\n", "#".repeat(level as usize), body)?,
+            Heading { level, body } => write!(out, "{} {}\n", "#".repeat(*level as usize), body)?,
             ListItem(body) => write!(out, "* {}\n", body)?,
             Quote(body) => write!(out, "> {}\n", body)?,
         };
@@ -166,6 +185,47 @@ impl Node {
     pub fn blank() -> Node {
         Node::Text("".to_string())
     }
+
+    /// Cheaply estimate the length of this node
+    ///
+    /// This measures length in bytes, *not characters*.  So if the user includes
+    /// non-ascii characters, a single one of these characters may add several bytes to
+    /// the length, despite only displaying as one character.
+    ///
+    /// This does include any newlines, but not any trailing newlines.  For example, a
+    /// preformatted text block containing a single line reading "trans rights! 🏳️‍⚧️"
+    /// would have a length of 30:  3 backticks, a newline, the text (including 16 bytes
+    /// for the trans flag), another newline, and another 3 backticks.
+    ///
+    /// ```
+    /// # use gemtext::Node;
+    /// let simple_text = Node::Text(String::from("Henlo worl"));
+    /// let linky_link = Node::Link { to: "gemini://cetacean.club/maj/".to_string(), name: Some("Maj".to_string()) };
+    /// let human_rights = Node::Preformatted("trans rights! 🏳️‍⚧️".to_string());;
+    ///
+    /// assert_eq!(
+    ///     simple_text.estimate_len(),
+    ///     "Henlo worl".as_bytes().len()
+    /// );
+    /// assert_eq!(
+    ///     linky_link.estimate_len(),
+    ///     "=> gemini://cetacean.club/maj/ Maj".as_bytes().len()
+    /// );
+    /// assert_eq!(
+    ///     human_rights.estimate_len(),
+    ///     "```\ntrans rights! 🏳️‍⚧️\n```".as_bytes().len()
+    /// );
+    /// ```
+    pub fn estimate_len(&self) -> usize {
+        match self {
+            Self::Text(text) => text.len(),
+            Self::Link { to, name } => 3 + to.as_bytes().len() +
+                name.as_ref().map(|n| n.as_bytes().len() + 1).unwrap_or(0),
+            Self::Preformatted(text) => text.as_bytes().len() + 8,
+            Self::Heading { level, body } => *level as usize + 1 + body.as_bytes().len(),
+            Self::ListItem(item) | Self::Quote(item)=> 2 + item.as_bytes().len(),
+        }
+    }
 }
 
 pub fn parse(doc: &str) -> Vec<Node> {