Add btree_gist support for enum types.

This will allow enums to be used in exclusion constraints.

The code uses the new CallerFInfoFunctionCall infrastructure in fmgr,
and the support for it added to btree_gist in commit 393bb504d7.

Reviewed by Tom Lane and Anastasia Lubennikova

Discussion:  http://postgr.es/m/56EA8A71.8060107@dunslane.net