Moved old miscellaneous benchmarks from std::mem

I have moved the benchmarks to test/bench/basic_benches.rs

These benchmarks did not belong to std::mem. They where moved there when
std::util was removed.

Additionaly, I have fixed a typo in the documentation of std::mem, and
refactored a function to use match instead of if.
Added run_flags directive to `compiletest`

Now it is possible to specify run-flags in tests. For example, by using `run-flags: --bench` the Bencher is run.