Expose some functions in phf_mac

This is useful for https://github.com/reem/rust-http-content-type/pull/7 and for any other syntax extensions which need to generate phf collections.

We generate a PhfMap based on data which is downloaded from the internet. Because the information is already parsed, it would be nice to use `create_map` directly.
Tests are failing because you removed the first set of `::` before `phf::PhfMap`.