Add new conj behaviour and better filter type for (filter identity coll)
Remove debugging prints
Better type for nth method.
WIP test for nth method
Improvements to nth special case.

Still needs more work recognising CountRange types and
when nth returns non-nil.
Revert IPColl changes. Tidy up filter/negation work.
Merge branch 'experimental'

Conflicts:
	src/test/clojure/clojure/core/typed/test/core.clj
Prepare for 0.1.18