Add COLUMN_TYPE_STRING function

Which returns the formatted type string for a given column. In the spirit of FORMAT_TYPE from Postgres, but that name seemed a little vague to me.

Quite a bit of flexibility in the overloads (1, 2 or 3), mostly for convenience. The one arg is for a single string containing period separated identifiers, as in the Sequence value functions. Add a general helper in Strings for parsing it but don't bother with handling quoted/casing at the moment (pre-existing task for that).