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
DOC/REL: adapt make.py file to upload with other user than pandas
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)
Documentation fix
Merge pull request #8598 from artemyk/io_doc_fix

Function pointer misspecified in io documentaion
Use '+' to qualify memory usage in df.info() if appropriate

This should resolve @kay1793's complaint in #8578.
Merge pull request #8599 from shoyer/qualify-mem-usage

Use '+' to qualify memory usage in df.info() if appropriate
Fix typo in visualization.rst doc
Merge pull request #8583 from jorisvandenbossche/doc-upload

DOC/REL: adapt make.py file to upload with other user than pandas
DOC: reorg whatsnew files to subfolder
Merge pull request #8604 from aisipos/fixplotdoctypo

Fix typo in visualization.rst doc
Merge pull request #8606 from jorisvandenbossche/doc-whatsnew-reorg

DOC: reorg whatsnew files to subfolder
DOC/REL: clean-up and restructure v0.15.0 whatsnew file (GH8477)