PERF/CLN: see what we can use from offsets.pyx
xref #11205 

https://github.com/pydata/pandas/blob/master/pandas/src/offsets.pyx

is in the repo but is not included in the build, nor updated in 2+ years.

Looks like their might be some cython code to speed up `DateOffset` apply in the non-vectorized cases.
Further might look to move _some_ routines from `tslib.pyx` for these types of things (and obviously would need to be included in the build).
