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:
	lib/Kitten/Interpret.hs
	lib/Kitten/Interpret/Monad.hs
	src/Main.hs
	src/Repl.hs
WIP

Done:

 * 'Name' -> 'Id'
 * 'NameMap' -> 'IdMap'
 * Use 'Text' instead of 'Id' for 'Def' names.
 * Start getting interactive mode working.

To do:

 * Make definitions persist properly across interactive fragments.