Allow infix type constructors

This commit adds infix type constructors (but not yet class constructors).
The documentation describes what should be the case.  Lots of tiresome
changes, but nothing exciting.

Allows infix type constructors everwhere a type can occur, and in a data
or type synonym decl.  E.g.

	data a :*: b = ....


You can give fixity decls for type constructors, but the fixity decl
applies both to the tycon and the corresponding data con.