Get rid of ParseState.p_value_substitute; use a columnref hook instead.

I noticed that p_value_substitute, which is a single-purpose kluge I added
in 2002 (commit b0422b21), could be replaced by having domainAddConstraint
install a parser hook that looks for the name "value".  The parser hook
code only dates back to 2009, so it's not surprising that we had to kluge
this in 2002, but we can do it more cleanly now.