6529 - don't let `addr`s escape functions

I've gone back and forth on this. I initially disallowed this, then allowed
it because I forgot why I disallowed it. The reason to disallow it: if
you return an `addr` to a variable allocated on the stack, the space might
be reused for a different type, which violates type-safety. And once you
can reinterpret bits of one type as another you lose memory-safety as well.

This has some interesting implications for Mu programs; certain kinds of
helper functions become impossible to write. Now I find myself relying a
lot more on scopes (and editor folding support) for abstracting details.
And they won't help manage duplication. We'll see how this goes.

While I'm being draconian about `addr`s on the stack, I'm still abusing
`addr`s on the heap, with the expectation that future checks on reclamation
will protect me. The boon and bane of stack space is that it's constantly
reclaimed.

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