POC: move to_offset to libfrequencies
The runtime non-cython import of to_offset is a sticking point in a bunch of tslibs code.  The reason we haven't moved to_offset up into cython is because it requires all of tseries.offsets, and we haven't wanted to move all of that up.

This moves to_offset up by defining appropriate dictionaries in the cython module and then filling them in the python modules.  It's a bit roundabout, but may be worthwhile (about to run asvs)

The refactor that was left out of this PR is moving tslibs.resolution.Resolution into tslibs.frequencies to avoid a circular/runtime import.