343 - experiment: rewrite rules

We'll worry later about implementing them in mu.
344 - about to give up on rewrite rules

I wanted to come up with some way to rewrite 'def-clause foo' to 'after
foo/more-clauses', something like:

  rewrite def-clause [
    (fn-name string-address) <- arg
    (label-name string-address) <- strcat (fn-name string-address deref) ("/more-clauses" literal)
    (body expr) <- arg
    reply `(after ,label-name ,body)
  ]

But the quasiquote is still a nested expression that doesn't fit our
model well.

Still an open question how to do template interpolation in mu.
345
346
347 - allow 'arg' channel to be rewound
348
349 - jump to label
350