-------------------------------
	Improve pattern type-signatures
	-------------------------------

The main effect of this commit is to implement the idea (originally
Marcin's suggestion) that type variables in pattern type signatures
are simply names for types; they don't have to name a type that is
itself a type variable.

For example

	f :: Int -> Int
	f (x::a) = let  y::a
			y = x
		   in x+y

is fine.  Here 'a' is a name for the type 'Int', and does not have
to be universally quantified.


I also took the opportunity to modularise the implementation of
pattern type-checking, mainly in TcMatches.  As a result pattern type
signatures should work in do-notation (which they didn't before).

ToDo: update documentation