Allow tailoring of ICU locales with custom rules

This patch exposes the ICU facility to add custom collation rules to a 
standard collation.  This would allow users to customize any ICU 
collation to whatever they want.  A very simple example from the 
documentation/tests:

CREATE COLLATION en_custom
     (provider = icu, locale = 'en', rules = '&a < g');

This places "g" after "a" before "b".  Details about the syntax can be 
found at 
<https://unicode-org.github.io/icu/userguide/collation/customization/>.

The code is pretty straightforward.  It mainly just records these rules 
in the catalog and feeds them to ICU when creating the collator object.