Control ctype behavior internally with a method table.

Control ctype behavior internally with a method table.

Previously, pattern matching and case mapping behavior branched based
on the provider. Refactor to use a method table, which is less
error-prone.

This is also a step toward multiple provider versions, which we may
want to support in the future.

Reviewed-by: Andreas Karlsson <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Discussion: https://postgr.es/m/2830211e1b6e6a2e26d845780b03e125281ea17b.camel%40j-davis.com