pltcl/plython fixes for spi_prepare types

Here's a patch along the same lines as the fix for plperl committed
earlier today, that allows passing type aliases to spi_prepare as well
as types named in pg_type. It also removes the mention of the previous
limitation in the pltcl docs. Unlike the plperl and pltcl cases, I
didn't use the simpler form that Tom suggested for plpython, as that
code wants to get hold of the HeapTuple. If anyone wants to tidy that up
some, feel free. Also, some regression tests from those with more tcl-fu
or python-fu that I have would be nice.

I'll apply this in a day or two unless there's an objection.

cheers

andrew