[ new ] Hereditary Subst. for STLC, Formalized

An Agda file corresponding to the normalization procedure described
in Keller and Altenkirch's 2010 paper on Hereditary Substitution.

Unfortunately _[_∷=_]Nf cannot be defined in terms of _[_∷=_]Ne which
in turn would be defined in terms of _[_∷=_]Var if we want Agda's
termination checker to be happy...