* Remove hashbrown and use BTree{Map,Set} from the alloc crate
wasmi-validation must handle untrusted input and hence we switch from
Hash{Set,Map} (whether std's or hashbrown's) to BTree{Set,Map} to avoid
algorithmic complexity attacks while retaining no_std support.
Closes#183
* Improve memory locality of checking for duplicate exports
Using a sorted slice gives us the same O(N log N) worst case execution
time as using a BTreeMap, but using a single allocation as with HashMap,
so that we should see better memory locality and hence better constant
factors when checking for duplicate exports.
8dac328ea7