Support benchmarking mmap feature

[brannondorsey]

Adds support to run cargo bench --features=mmap, using open_mmap() in place of open_readfile() when this is the case.

# To test
cargo bench && cargo bench --features=mmap

Results are comparable on my machine, which is perhaps to be expected. Nonetheless this seems like a useful improvement to benches/lookup.rs.

[oschwald]

Thanks!