pgsql: Improve make_greater_string() with encoding-specific incrementer

Improve make_greater_string() with encoding-specific incrementers.

This infrastructure doesn't in any way guarantee that the character
we produce will sort before the one we incremented; but it does at least
make it much more likely that we'll end up with something that is a valid
character, which improves our chances.

Kyotaro Horiguchi, with various adjustments by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/78d523b633d0c251c904318f2ba9916e2c47b9e8

Modified Files
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src/backend/utils/adt/selfuncs.c | 65 ++++++----
src/backend/utils/mb/wchar.c | 257 ++++++++++++++++++++++++++++++++++++++
src/include/mb/pg_wchar.h | 3 +
3 files changed, 297 insertions(+), 28 deletions(-)