Along the way, this commit also rationalizes the traits a bit more.
Moving to stable required vendoring a couple of minor things (notably,
radix formatting).
Now, arithmetic binary operator traits take operands by value,
but non-copyable types such as `BigUint` should not always moved out when
applying operators.
This commit implements these operators not only for bare structs also
these references.
By-value implementations are forwarded to by-reference implementations
for now. In the future, by-value implementations may be replaced with
more efficient implementations (for example, the implementation that
reuses moved buffers.)
Aaron Turon
Alex Crichton
Huon Wilson
gifnksm
Sven Nilsen