401 - stop abbreviating ops

We expect users to come across mu from arbitrary bits of code, so try to
make each line as self-contained as possible.
402
403 - 'function' is more clear than 'def'
404
405 - permit loading just low levels of codebase

When I'm doing extensive surgery to the internals I want to avoid
loading higher levels; they aren't expected to work. But I don't want to
keep different levels in separate files just for that. And I definitely
don't want to put low-level stuff first. Now I can influence loading in
a cross-cutting manner by creating sections with numbers:

  (section 100
    ...code...)

And disabling them by running:

  $ ./anarki/arc 99 mu.arc.t

Currently we load all mu 'system software' in level 100, so running at
level 99 sidesteps them. Lower levels coming soon.

But most of the time we don't need to worry about levels, and the 'mu'
script lets us forget about them. Just run .mu files with:

  $ ./mu factorial.mu

To run tests:

  $ ./mu test mu.arc.t
406 - mu tests run at level 10

We're now going to start doing surgery on low-level helpers.
407
408

Within a level we can now just rely on simple ordering.
409 - new arg representation

Tests only passing at level 9:
  $ arc load.arc 9 mu.arc.t
410
411 - 'tokenize-args' must handle blocks
412 - tests passing at level 10
413
414
415
416 - renumber layers, but still passing until layer 10

  $ arc load.arc 10 mu.arc.t
417 - partially through layer 11
418 - more progress in layer 11

With the notion of layers we add the constraint of only being able to
formulate loop tests without 'run' -- otherwise they'd have to leave
layer 11 because they have so many more dependencies.

Might bring back the run-based tests afresh later. Especially since
loop-fail test is now totally gone.
419 - layer 11 now passing
420 - no unnecessary structure in tests

It makes them harder to appreciate.