Support Unicode full case mapping and conversion.

Support Unicode full case mapping and conversion.

Generate tables from Unicode SpecialCasing.txt to support more
sophisticated case mapping behavior:

 * support case mappings to multiple codepoints, such as "ß"
   uppercasing to "SS"
 * support conditional case mappings, such as the "final sigma"
 * support titlecase variants, such as "dž" uppercasing to "DŽ" but
   titlecasing to "Dž"

Discussion: https://postgr.es/m/ddfd67928818f138f51635712529bc5e1d25e4e7.camel@j-davis.com
Discussion: https://postgr.es/m/27bb0e52-801d-4f73-a0a4-02cfdd4a9ada@eisentraut.org
Reviewed-by: Peter Eisentraut, Daniel Verite