Add non-recursive let-bindings for types
This patch adds to Core the ability to say let a = Int in <body> where 'a' is a type variable. That is: a type-let. See Note [Type let] in CoreSyn. * The binding is always non-recursive * The simplifier immediately eliminates it by substitution So in effect a type-let is just a delayed substitution. This is convenient in a couple of places in the desugarer, one existing (see the call to CoreTyn.mkTyBind in DsUtils), and one that's in the next upcoming patch. The first use in the desugarer was previously encoded as (/\a. <body>) Int rather that eagerly substituting, but that was horrid because Core Lint had do "know" that a=Int inside <body> else it would bleat. Expressing it directly as a 'let' seems much nicer.