Implement parameter defaults and slurpy parameters in 007 itself

[masak]

The original idea was that we'd be able to do this complete from the "outside", in userland, by providing a custom binder. (See #112.) That's still a possibility... but as a stepping stone we can simply model things in the implementation without exposing them in the language syntax, and then switching on the proper syntax in a 007-level module (which we can also fake as a pragma/flag initially).

I didn't include optional parameters in the list of things we should model, because when I think about it, an optional parameter is simply a parameter with a default None.

Also, worth keeping in mind that in the general case, a parameter default is a closure, not a value. The closure should close over the parameters declared so far.

Among the builtins,

• prompt could benefit from having its msg parameter default to ""
• substr could benefit from having its chars parameter default to self.chars
  • (showing that the parameter default ought to be a closure that closes on self)
  • obviating prefix and suffix on Str — see Make substr not have an optional argument #113 — though we might decide to keep them around
• say could benefit — aw, that'd be nice! — from having a slurpy parameter

As part of this, we might want to finally separate a Parameter type from Q::Parameter, which is a bit misplaced in Sub, to be honest. (Cf #253.)

(I almost labeled this one "low-hanging fruit", but it might have unintended consequences, who knows.)

[masak]

I realize I would also like concat to be a builtin function (again), rather than a method on Array. The main reason is that I would prefer it to take a variable number of arrays (or rather iterables), and then it looks kind of lopsided to have it as a method on the first array.

Kthx.