pgsql: Refactor string case conversion into provider-specific files.

Refactor string case conversion into provider-specific files.

Create API entry points pg_strlower(), etc., that work with any
provider and give the caller control over the destination
buffer. Then, move provider-specific logic into pg_locale_builtin.c,
pg_locale_icu.c, and pg_locale_libc.c as appropriate.

Discussion: https://postgr.es/m/7aa46d77b377428058403723440862d12a8a129a.camel@j-davis.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/86a5d6006aff956a5e00982b7628177fa7dc5027

Modified Files
--------------
src/backend/utils/adt/formatting.c        | 465 ++++--------------------------
src/backend/utils/adt/pg_locale.c         |  78 +++++
src/backend/utils/adt/pg_locale_builtin.c |  80 +++++
src/backend/utils/adt/pg_locale_icu.c     | 130 ++++++++-
src/backend/utils/adt/pg_locale_libc.c    | 327 +++++++++++++++++++++
src/include/utils/pg_locale.h             |  14 +-
6 files changed, 676 insertions(+), 418 deletions(-)