Add Unicode property tables.

Add Unicode property tables.

Provide functions to test for Unicode properties, such as Alphabetic
or Cased. These functions use tables derived from Unicode data files,
similar to the tables for Unicode normalization or general category,
and those tables can be updated with the 'update-unicode' build
target.

Use Unicode properties to provide functions to test for regex
character classes, like 'punct' or 'alnum'.

Infrastructure in preparation for a builtin collation provider, and
may also be useful for other callers.

Discussion: https://postgr.es/m/ff4c2f2f9c8fc7ca27c1c24ae37ecaeaeaff6b53.camel%40j-davis.com
Reviewed-by: Daniel Verite, Peter Eisentraut, Jeremy Schneider