Split unicode case LUTs in single and multi variants

The majority of char case replacements are single char replacements,
so storing them as [char; 3] wastes a lot of space.

This commit splits the replacement tables for both `to_lower` and
`to_upper` into two separate tables, one with single-character mappings
and one with multi-character mappings.

This reduces the binary size for programs using all of these tables
with roughly 24K bytes.

Author: martingms