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
Also add comments reminding that IEEE 754 requires unusual semantics for
comparison operators as applied to NaNs (x != x, if x = NaN), in case someone
in the future wants to get clever.
API is (for now) mostly by value, there are options to use it by
reference if you like. Hash and equality functions must be pure
and by reference (forward looking to the day when something
like send_map becomes the standard map).
