Turns callback-based functions into channels, with the error
propagation of `jtk-dvlp.async`.

A callback API does not fit into a sequence of steps: the rest of
the work has to move inside the callback, and every further call
nests one level deeper. `cb->c` turns such a call into a channel, so
it reads like any other step in a go block — and a failure arrives
as a thrown error rather than as a second callback.

WATCHOUT: `cb->c` is a macro that *rewrites* the expression handed
to it. It looks for the symbols `callback`, `resolve` and `reject`
inside and puts its own functions in their place. Those symbols are
therefore written bare, without ever being defined — and they are
only found where they literally stand. Move a callback into a helper
function and the macro cannot see it any more.

Bridges promises and channels, in both directions, with the error
propagation of `jtk-dvlp.async`.

A rejected promise arrives on the channel as a carried error and is
thrown by `<!`; an error carried on a channel rejects the promise it
is turned into. So a `.then`/`.catch` chain and a go block stack can
be mixed without either side losing an error.

WATCHOUT: `promise` means different things on the two platforms. In
ClojureScript it is a `js/Promise` with `then` and `catch`. On the
JVM it is a `clojure.core/promise`, which has neither and only takes
a single `deliver` — there is no rejection there, only a delivered
exception value. The functions here even that out; where it still
shows through, it is said so.