Remove lookupVariable()
Make checkVariable() return checked value.
Stop exporting Interpreter.checkVariable()
Make List data public.

* So cloner can get at it in future commit.
Merge branch 'master' into clone-env

Conflicts:
	src/library.js
Remove merge.
Put Generic fns on proto.
Stop getInitialEnv taking extraTypes and initalCtx
Set constructor on Generic and List prototypes.

* For testing.
Export Generic from Library.
Allow env to be cloned.
Add tests for cloning.
Minify.
Make clone() clone fns.
Add tests to check ctx types and fns get cloned.
Check clone env doesn't pass an added vars to orig
Minify.
Remove clone() methods from List and Generic

* Keep all cloning together for readability and interface hiding.
Make cloning preserve references between objects.
Test references preserved when cloning.