CREATE FUNCTION support

[fulghum]

Doltgres does not yet support user-defined functions created through CREATE FUNCTION statements.

Example from Sakila data dump:

CREATE FUNCTION _group_concat(text, text) RETURNS text
    AS $_$
SELECT CASE
  WHEN $2 IS NULL THEN $1
  WHEN $1 IS NULL THEN $2
  ELSE $1 || ', ' || $2
END
$_$
    LANGUAGE sql IMMUTABLE;

Discovered through testing the Sakila data dump.