Finish 1.0.8
Speed up access to RDF::x URI constants.

Previously accessing these unnecessarily created a new (interned) URI,
which is relatively slow.
Make literal initialisation a little faster.

This is mainly from comparing datatypes by string rather than by URIs,
which are slow to initialise.
Add Tom Nixon to credits.
Merge branch 'tomjnixon-speedup_literal'
This closes issue #129