New let/letrec syntax

[rolyp]

For various reasons I think I prefer the syntax:

   Let ::= let Var = Exp
   RecDef ::= fun ...
   LecRec ::= letrec RecDef (; RecDef)*
   Exp ::= ... | (Let | LetRec)* in Exp

to the current:

   Let ::= let Var = Exp in Exp
   RecDef ::= fun ...
   LecRec ::= letrec RecDef (; RecDef)* in Exp
   Exp ::= ... | Let | LetRec

In particular the prelude (later, some more realistic form of module) can be of the form (Let | LetRec)* rather than RecDef (; RecDef)* as it is currently.