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		Towards kinded data type decls
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Move towards being able to have 'kinded' data type decls.
The burden of this commit, though, is to tidy up the parsing
of data type decls.  Previously we had

	data ctype '=' constrs

where the 'ctype' is a completetely general polymorphic type.
	forall a. (Eq a) => T a

Then a separate function checked that it was of a suitably restricted
form.  The reason for this is the usual thing --- it's hard to tell
when looking at

	data Eq a => T a = ...

whether you are reading the data type or the context when you have
only got as far as 'Eq a'.

However, the 'ctype' trick doesn't work if we want to allow

	data T (a :: * -> *) = ...

So we have to parse the data type decl in a more serious way.
That's what this commit does, and it makes the grammar look much nicer.
The main new producion is tycl_hdr.