Catch async errors

[nilern]

Fixes #11

• Typically async libs complect an error semi-monad with their async semi-monad (JS Promise, Promesa, Manifold). This error handling is essentially async exceptions, and not hooking into it means that we only handle exceptions and put them into :error in synchronous usage.
• This branch adds a catch method to the AsyncContext protocol, which maps directly to the aforementioned async library error handling.
• Some async abstractions like futures and core.async channels have no such error handling strategy. The proposal here is to treat the future or channel yielding an Exception/js/Error as an async throw. This could be removed if the jury is so inclined, making the AsyncContext/catch implementation on those types a no-op.
• A lot of tests are included here, but they are simply analogous to the sync exception catching tests.
• BTW removes the Figwheel test runner obsoleted by #22.

[arichiardi]

Checking :smile:

[arichiardi]

Looks great to me, thanks for tackling this!

[miikka]

Let's have it, then.