7.3x speedup in string map search speed on a microbenchmark of pure hashmap
searching against a constant string, due to the lack of allocations.
I ran into a few snags.
1. The way the coherence check is set up, I can't implement `Equiv<@str>` and
`Equiv<~str>` for `&str` simultaneously.
2. I wanted to implement `Equiv<T>` for all `T:Eq` (i.e. every type can be
compared to itself if it implements `Eq`), but the coherence check didn't
like that either.
3. I couldn't add this to the `Map` trait because `LinearMap` needs special
handling for its `Q` type parameter: it must not only implement `Equiv<T>`
but also `Hash` and `Eq`.
4. `find_equiv(&&"foo")` doesn't parse, because of the double ampersand. It has
to be written `find_equiv(& &"foo")`. We can probably just fix this.
Nevertheless, this is a huge win; it should address a major source of
performance problems, including the one here:
http://maniagnosis.crsr.net/2013/02/creating-letterpress-cheating-program.html
