replace the core-map benchmark

* Closes #4603
auto merge of #5476 : thestinger/rust/bench, r=graydon

The old string benchmarks weren't very useful because the strings weren't long enough, so I just threw those out for now. I left out benchmarks of `oldmap` because it's clear that it's 30-40% slower and it doesn't implement the `Map` trait.

This also cleanly divides up `insert`, `search` and `remove`.