Add support for Unicode whitespace in [:blank:]

Add support for Unicode whitespace in [:blank:]

See Bug#25366.

* src/character.c (blankp): New function for checking Unicode
horizontal whitespace.
* src/regex.c (ISBLANK): Use 'blankp' for non-ASCII horizontal
whitespace.
(BIT_BLANK): New bit for range table.
(re_wctype_to_bit, execute_charset): Use it.
* test/lisp/subr-tests.el (subr-tests--string-match-p--blank): Add
unit test for [:blank:] character class.
* test/src/regex-tests.el (test): Adapt unit test.
* doc/lispref/searching.texi (Char Classes): Document new Unicode
behavior for [:blank:].