Tweak parser so that there is a defined representation for datatypes

Tweak parser so that there is a defined representation for datatypes
bpchar, bit, numeric with typmod -1.  Alter format_type so that this
representation is printed when the typmod is -1.  This ensures that
tables having such columns can be pg_dump'd and reloaded correctly.
Also, remove the rather useless and non-SQL-compliant default
precision and scale for type NUMERIC.  A numeric column declared as
such (with no precision/scale) will now have typmod -1 which means
that numeric values of any precision/scale can be stored in it,
without conversion to a uniform scale.  This seems significantly
more useful than the former behavior.  Part of response to bug #513.