Using Data.RDF.Types for QueryGenerator, String -> Text and more

No more home-brewn RDF types in HSparql, Data.RDF.Types (from RDF4H) are reused.
Another one major change that should be noted is the switch String -> Text in the output. I guess you'll have to bump a middle version number to indicate the break of backward compatibility.

I'm going to introduce String -> Text switch in "Connection.hs" a bit later - it seems that it's going to be a messy refactoring ;-)

UPDATE: Actually, forget the last line. It appears that all String-related stuff is not exported and pretty much serves as a glue for the String-ish output of XML parser. So, you can either roll out a last standalone version of HSparql, or wait until the merge with RDF4H.