boehm_gc for Rust

This crate is a prototype that provides an allocator to Rust that has the ability to root garbage-collected pointers allocated by the Boehm GC.

The ability to swap out the Rust allocator is a very new feature that will require a recently nightly to take advantage of. I've done a few tests on this crate and it does work - pointers allocated by boehm_gc::gc_allocate are rooted by anything that is allocated by the system allocator used by std.

This crate does require that you have libgc installed on your system. It can usually be obtained through your package manager of choice (I used homebrew on OSX just fine).