MAINT: statistics review: remove unused functions in stats._support

Closes gh-708
Closes gh-710
Closes gh-711
Closes gh-712
MAINT: rewrite and test stats.itemfreq.  Complete removal of stats._support

Closes gh-599 (stats.itemfreq statistics review)
Closes gh-706 (stats.abut statistics review - removed)
Closes gh-707 (stats.unique statistics review - removed)
Added tests for abcd_normalize
See #1973.
Rewritten function abcd_normalize, closes #1973
Code is notably simplified, at the cost of not showing specific
error messages for each shape mismatch. That is easy to fix, but
I wanted to leave this state here.
Fixed several corner cases
Improved error message for bad input array shapes
Merge pull request #2756 from Juanlu001/fix-1973

Rewritten abcd_normalize, fix 1973
Merge pull request #2778 from rgommers/stats-cleanup

Clean up stats._support, close statistics review issues