feat(parse,interp,e2e): Add basic global native functions

[sjbarag]

This gets the groundwork set up for the rapid implementation of global functions — including the ability to call them at runtime. There's a few caveats there for now though:

1. Only LCase, UCase, (string lowercase and string uppercase) and RebootSystem (which does nothing in this implementation [1]) are implemented so far.
2. I haven't figured out how I want to implement function argument type checking, e.g. causing an error to occur when LCase(185) is called.
3. Adding new globally-scoped functions is probably a bit tougher than it needs to be right now. That's pretty minor though.

[1] I guess I could shell out to shutdown -r but that seems particularly mean and useless.

[sjbarag]

Oh right, I forgot to mention:

The same Callable system will work for user-defined functions as well! We still need to parse and interpret them, but the overall pattern will still apply 😄