Subtype improvements

Today's update to the DBMS corrects the handling of the new URI-based subtypes of standard types. As mentioned in previous posts, these are a novelty in Pyrrho, allowing declarations such as
create type uk_numberplate as char with 'http://dvla.gov.uk'
As of today's version the representation clause ("as char" in this example) for such types is constrained to be a standard type. Subtypes of such types must be declared as follows:
create type sc_nunberplate under uk_numberplate as char with 'http://dvla.scotland.gov.uk'
The semantic of IS OF for these subtypes has been corrected.