TST/PERF: optimize tm.makeStringIndex

* add tm.rands_array(nchars, size) function to generate string arrays

* add tm.randu_array(nchars, size) function to generate unicode arrays

* replace [rands(N) for _ in range(X)] idioms:
  - with makeStringIndex in benchmarks to maintain backward compatibility
  - with rands_array in tests to maintain 1-to-1 type correspondence
BUG: dtype query with negation bug (GH8568)
DOC: updates release and v0.15.0.txt for release
DOC: fix example of GH8512 name clash with following examples
Merge pull request #8579 from jorisvandenbossche/doc-fixes015

DOC: fix example of GH8512 name clash with following examples
RLS: 0.15.0 final
DOC: start 0.15.1 cycle for docs
Merge pull request #8575 from immerrr/optimize-makestringindex

TST/PERF: optimize tm.makeStringIndex
ENH: Allow columns selection in read_stata

Allow columns to be selected when importing Stata data files.

Closes #7935
Merge pull request #8577 from bashtage/stata-columns

ENH: Allow columns selection in read_stata
BUG: invalid import checking in gbq (GH8587)