Implement libinternals

Author: Brock Mendel <jbrockmendel@gmail.com>

Closes pandas-dev#19293 from jbrockmendel/libinternals and squashes the following commits:

f0146b2 [Brock Mendel] fix missing cimport
70514ce [Brock Mendel] revert edits to be pure cut/paste
0a94cca [Brock Mendel] Cleanup leftover cimport, use c-api funcs for isinstance
3fb0d0a [Brock Mendel] implement libinternals

Author: jbrockmendel