This is a well-worn idea: use a `promise-channel` of `[result nil]` or
`[nil error]` pairs. The `go-pair` and `<?` macros handle catching
exceptions (important, given that synchronous CLJ code expects to throw
rather than return an error promise or similar), allowing code like:
```
(go-pair
(let [result (<? (pair-chan-fn))]
(when (not result)
(throw (Exception. "No result!")))
(transform result)))
```
to be expressed naturally. These are the equivalents of `async` and
`await` in JS.
The implementation is complicated by significant incompatibilities
between CLJ and CLJS. The solution presented here takes care to
separate the macro definitions into CLJ. Sadly, this requires
namespacing the per-environment symbols explicitly; but we hope to
minimize such code in files like this.
The most significant restriction to this approach is that consumers must
require the transitive dependencies of the macro-defining modules. See
the included tests (both CLJ and CLJS) for the appropriate
incantations (for pair-chan, core.async, and test).
