DOC, ENH, TST: Speed up dict sampling by creating a lookup table.
Instead of doing a binary-search on every sample, it's an array index now.
Runtime from 2.7s -> .3s for 1mil samples.
Fix resample for PEP 287.
Add more unit tests for the class distributions from proba=dict.
BUG: Fix redundant shuffle. Add unit tests for shuffles.