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get rid of substitution in Case (use lambdas); introduce Match as derived form that involves binders

We no longer abuse empty strings for anonymous binders. Instead, we
now have a data type for binders: a binder is either named or
anonymous.

Also, make our redefinition of done more robust under different
orders of Importing modules.

tests.v is temporarily broken

Before they were not, for example:

  Check ('10 + '10 )%L.           (* prints ('10 + '10)%L *)
  Eval simpl in ('10 + '10 )%L.   (* prints (Lit 10 + Lit 10)%L *)

The notation added by this comment is ambigious, for example the
notation '10 + '10 is used for both:

  BinOp PlusOp (Lit (LitNat 10)) (Lit (LitNat 10))
  BinOp PlusOp (of_val (LitV (LitNat 10))) (of_val (LitV (LitNat 10)))

But fortunatelly, these terms are convertible.

Note that literals 'x are now parsed as values (as a LitV), but still
pretty printed when they appear as expressions (as a Lit).