Layers in the 3xx series use all the available syntax sugar for SubX programs.
They're used by the Mu translator (apps/mu.subx) and can also be called from
Mu programs if they meet certain criteria:

  - There's a signature for them in 400.mu
  - Inouts on the stack, outputs in registers
  - Valid Mu types everywhere (Mu's type system isn't expressive enough for
    everything SubX does in rare situations.)
  - No way to for an `addr` to escape a function. No `(... addr ... addr ...)`
    inouts, and no `(... addr ...)` outputs.

While functions _can_ be called, not all SubX functions meeting these criteria
_should_ be called. In particular, avoid exporting functions that could be
misused. A classic example is trying to add a `size-of` operator. If you're
doing that you're likely going to rely on programmers to use it correctly. Mu
tries to be idiot-proof. Even if SubX requires greater care, using SubX
primitives from Mu should not.