Better TH -> HsSyn conversion

	Merge to stable (attempt)

This commit monad-ises the TH syntax -> HS syntax conversion.
This means that error messages can be reported in a more civilised
way.  It also ensures that the entire structure is converted eagerly.
That means that any exceptions buried inside it are triggered 
during conversion, and caught by the exception handler in TcSplice.
Before, they could be triggered later, and looked like comiler
crashes.