This allows implementing e.g. the Fibonacci sequence in a typable
fashion.

This includes a nix expression for an interactive GHOTL REPL, including
readline goodness (although I sort of cheated for that).

The evaluator is now more type-safe in that terms that can not be
evaluated any further get their own type, allowing neater distinctions
and catching a few cases where evaluation might loop infinitely more
easily.

The new evaluator steps more neatly, and indicates progress or the next
step through a dedicated type.

This is achieved by adding constraints to schemes. I'm not 100% sure
this is correct yet, need to try applying it some more.

SKK will not necessarily evaluate to id, since it is a function and we
evaluate lazily; we have to apply it to something instead.

Less parentheses make stuff clearer