This reverts commit 17e0070df93f4262908f884dca4b08cb7d0bba7f.

Remove traces of the three removed bit generators
Add lock to Cython examples

remove numpy.random.gen, BRNG.generator, pcg*, rand, randn
remove use_mask and Lemire's method, fix benchmarks for PCG removal
convert brng to bitgen (in C) and bit_generator (in python)
convert base R{NG,andom.*} to BitGenerator, fix last commit
randint -> integers, remove rand, randn, random_integers
RandomGenerator -> Generator, more "basic RNG" -> BitGenerator
random_sample -> random, jump -> jumped, resync with randomgen
Remove derived code from entropy
Port over changes accepted in upstream to protect log(0.0) where relevant
fix doctests for jumped, better document choice
Remove Python 2.7 shims
Use NPY_INLINE to simplify
Fix performance.py to work
Renam directory brng to bit_generators
Fix examples wiht new directory structure
Clarify relationship to historical RandomState
Remove references to .generator
Rename xoshiro256/512starstar

remove files that were part of the origal repo
rework randomgen docs to integrate with numpy and fix some links
remove convenience functions, require explicit call to gen.brng
move code out of numpy.random.randomgen into numpy.random