This commit replaces the old yacc parser with a Happy-generated one.
Notes:

	- The generated .hs file is *big*.  Best to use a recent
	  version of Happy, and even better to add the -c flag
	  to use unsafeCoerce# with ghc (versions 4.02+ please).

	- The lexer has grown all sorts of unsightly growths and
	  should be put down as soon as possible.

	- Parse errors may result in strange diagnostics.  I'm looking
	  into this.

	- HsSyn now contains a few extra constructors due to the way
	  patterns are parsed as expressions in the parser.

	- The layout rule is implemented according to the Haskell
	  report.  I found a couple of places in the libraries where
	  we previously weren't adhering to this - in particular the
	  rule about "nested contexts must be more indented than
	  outer contexts".  The rule is necessary to disambiguate
	  in the presence of empty declaration lists.