API/ENH: add method='nearest' to Index.get_indexer/reindex and method to get_loc
Fixes #8845

Currently only an index method; I'm open to ideas for how to expose it more
directly -- maybe `df.loc_nearest`?.

In particular, I think this is usually a more useful way to do indexing for
floats than pandas's reindex based `.loc`, because floats are inherently
imprecise.

CC @jreback @jorisvandenbossche @immerrr @hugadams
