Tweak the API for per-datatype typmodin functions so that they are passed

Tweak the API for per-datatype typmodin functions so that they are passed
an array of strings rather than an array of integers, and allow any simple
constant or identifier to be used in typmods; for example
	create table foo (f1 widget(42,'23skidoo',point));
Of course the typmodin function has still got to pack this info into a
non-negative int32 for storage, but it's still a useful improvement in
flexibility, especially considering that you can do nearly anything if you
are willing to keep the info in a side table.  We can get away with this
change since we have not yet released a version providing user-definable
typmods.  Per discussion.