BUG: GH4017, efficiently support non-unique indicies with iloc

PERF: getting an indexer with a non_unique index, now MUCH faster

PERF: vbench for loc/iloc with dups

BUG: sparse reindex needed takeable arg

TST

BUG: correctly interpret tuple/list in non_unique indexers

BUG: df.loc[idx] with out-of-bounds indexers not correctly interpreted

PERF: df.loc with non-unique index not blazing fast!
Merge pull request #4018 from jreback/iloc_bug

BUG: GH4017, efficiently support non-unique indicies with iloc