Represented Peano numbers internally as Haskell integers; added a case for type checking them
Implemented evaluation and reification of successor and recursor for Peano numbers
Added support for the sucessor and recursor in both untyped and typed substitution
Revert "Systematically renamed `Nat' to `Int' and `nat' to `int' (fix #49)"
Implemented `safeSub' so that subtracting a larger natural number from a smaller one gets stuck
Adjusted tests of arithmetic operations