Remove a bunch of "extern Datum foo(PG_FUNCTION_ARGS);" declarations that
are no longer needed now that PG_FUNCTION_INFO_V1(foo) provides that.
Some of these were evidently missed in commit e7128e8dbb305059, but others
were cargo-culted in in code added since then. Possibly that can be blamed
in part on the fact that we'd not fixed relevant documentation examples,
which I've now done.
This provides a mechanism for specifying conversions between SQL data
types and procedural languages. As examples, there are transforms
for hstore and ltree for PL/Perl and PL/Python.
reviews by Pavel Stěhule and Andres Freund