Re: transforms

Here is an updated patch for the transforms feature. The previous
discussion was here:
http://www.postgresql.org/message-id/1339713732.11971.79.camel@vanquo.pezone.net

Old news: At the surface, this contains:

- New catalog pg_transform

- CREATE/DROP TRANSFORM

As proofs of concept and useful applications, I have included transforms for

- PL/Perl - hstore
- PL/Python - hstore
- PL/Python - ltree

New news: I have tried to address all issues raised during previous
reviews.

The regression tests for hstore_plpython and ltree_plpython don't pass
with Python 3. This needs to be fixed, obviously, but it's an issue
unrelated to the core functionality.

As a demo that this can be used externally as well, I have written a
proof-of-concept transform between plpython and the mpz type in the pgmp
extension (multi-precision arithmetic extension):
https://github.com/petere/pgmp/tree/transforms/plpython

Note: There was a lot of churn lately in src/backend/commands/, and I
had to fix merge conflicts several times, so you need source as of about
right now to apply this patch cleanly.

Side issue/peculiarity: I had to change some shared library linking
flags for OS X. Ordinarily, when linking a dynamically loadable module
on OS X, if there are unresolved symbols, it's an error. But for
example, when we link hstore_plpython, we expect some symbols to be
resolved in hstore.so or plpythonu.so, so we need to ignore unresolved
symbols. This change basically just makes OS X behave like other
platforms in this regard. There might be other portability issues on
other platforms.