Collation & ctype method table, and extension hooks

The attached patch series refactors the collation and ctype behavior
into method tables, and provides a way to hook the creation of a
pg_locale_t so that an extension can create any kind of method table it
wants.

In practice, the main use is to replace, for example, ICU with a
different version of ICU. But it can also be used to control libc
behavior, or to use a different set of methods that have nothing to do
with ICU or libc.

It also isolates code to some new files: ICU code goes in
pg_locale_icu.c, and libc code goes in pg_locale_libc.c. And it reduces
a lot of code that branches on the provider. That's easier to reason
about, in my opinion.

With these patches, the collation provider becomes mainly a catalog
concept used to create the right pg_locale_t, rather than an execution-
time concept.

We could take this further and make providers a concept in the catalog,
like "CREATE LOCALE PROVIDER", and it would just provide an arbitrary
handler function to create the pg_locale_t. If we decide how we'd like
to handle versioning, that could potentially allow a much smoother
upgrade process that preserves the provider versions.

Regards,
Jeff Davis