TYP: Index.join, get_indexer, reindex
@simonjayhawkins this is technically incorrect bc im specifying returned dtypes as np.intp.  Is there a valid way to do this?  In working in core.internals trying to figure out whether we want int64 or intp I had to track all of these down to be sure.  Would prefer to avoid re-doing this in a few weeks when I forget.