Literal parsing as a constraint

[wouterbeek]

If O in rdf(S, P, O) is an expensive literal (in terms of in_ground_type_hook/3 and out_type_hook/3) then it may be better to first match to the unparsed rdf_db term literal(type(D,Lex)) and only later, when first used, parse Lex into a Val.

This may be implemented as part of the rdf_where/1 or {...} notation.