WIP Improve interpreter speed by interpreting IR instead of AST.

Using the flattened representation and not being shy about IO was a
significant improvement, but there is still more work to be done.

TODO Reinstate REPL using new interpreter.

Conflicts:
	src/Main.hs
	src/Repl.hs
Kill debug messages.
Remove debug definitions.
Temporarily remove unused constructors.
Kill 'impure' builtin.
Fleshing out builtins.