pgsql: Refactor convert_case() to prepare for optimizations.

Refactor convert_case() to prepare for optimizations.

Upcoming optimizations will add complexity to convert_case(). This
patch reorganizes slightly so that the complexity can be contained
within the logic to convert the case of a single character, rather
than mixing it in with logic to iterate through the string.

Reviewed-by: Alexander Borisov <lex(dot)borisov(at)gmail(dot)com>
Discussion: https://postgr.es/m/44005c3d-88f4-4a26-981f-fd82dfa8e313@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d3b2e5e1ab5ca3e9738d20409debfdd299e65a2e

Modified Files
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src/common/unicode_case.c | 153 ++++++++++++++++++++++++++++++----------------
1 file changed, 101 insertions(+), 52 deletions(-)