Add LANGUAGE extensions for inferred signatures.

Some of the inferred signatures required LANGUAGE extensions that
were not enabled. Since now GHC checks that we have to explicitly
list required extensions.

(cherry picked from commit a6049abce040713e9a5f175887cf70d12b9057c6)

This should fix #25

Conflicts:
	Data/Vector/Generic/Mutable.hs
	Data/Vector/Mutable.hs