vec::reserve should reserve the exact amount requested

[brson]

This function currently reserves to the next power of two. This makes sense during compiler push operations, but the library function is typically called when the final size of the vector is known. Even when that assumption isn't correct we'll end up allocating to the next power of two on the next push.

[killerswan]

When this change is made, we should try out str::reserve again, too: https://gist.github.com/1661506

[killerswan]

At the moment, due to some unrelated error, I can't build, but this branch shows what I'm thinking will work: killerswan/rust@master...exact_reserve

[brson]

Fixed by the killerswan