Implement Categorical.searchsorted(v, side, sorter) #8420
closes #8420

Continuation of #8928

looks good. just need a release note, enhancements section. ping when you have pushed. thanks.
 can you add a release note?
 Added in whatsnew for 0.15.2
 merged via https://github.com/pydata/pandas/commit/5928075f58715207da9c21f03c9da9ea10d4082b

thanks!
