Re: psql: add \create_function command

\set fbody `cat source_file.txt`
CREATE FUNCTION foo() RETURNS whatever AS :'fbody' LANGUAGE ...;

and maybe we should say that that's sufficient.

  It's a bit
klugy though.  One level of improvement could be to get rid
of the dependency on "cat" by inventing a backslash command
to read a file into a variable:

\file_read fbody source_file.txt

(\file_write to go the other way seems potentially useful too.)

Or we could cut out the intermediate variable altogether
by inventing something that works like :'...' but reads
from a file not a variable.  That might be too specialized
though, and I'm not sure about good syntax for it either.
Maybe like

CREATE FUNCTION foo() RETURNS whatever AS :{source_file.txt} LANGUAGE ...;