Remove unused, avoid uses of deprecated api
`core.dtypes.common.is_period` is confusingly named and never used.  This PR gets rid of it.

The deprecated numpy API that causes zillions of warnings includes `ndarray.data`, so this replaces the old usage with the encouraged usage.

A bunch of `tslibs.util` isn't actually used in tslibs.util, so the relevant functions are moved up to `_libs.util`.

Docstrings for most of what's left in tslibs.util.

Simplify util._checknull by removing an unnecessary try/except.