If #[feature(default_type_parameters)] is enabled for a crate, then
deriving(Hash) will expand with Hash<W: Writer> instead of Hash<SipState> so
more hash algorithms can be used.
This leverages the new hashing framework and hashmap implementation to provide a
much speedier hashing algorithm for node ids and def ids. The hash algorithm
used is currentl FNV hashing, but it's quite easy to swap out.
I originally implemented hashing as the identity function, but this actually
ended up in slowing down rustc compiling libstd from 8s to 13s. I would suspect
that this is a result of a large number of collisions.
With FNV hashing, we get these timings (compiling with --no-trans, in seconds):
| | before | after |
|-----------|---------:|--------:|
| libstd | 8.324 | 6.703 |
| stdtest | 47.674 | 46.857 |
| libsyntax | 9.918 | 8.400 |
