Implement async generator combination functions similar to the ones from rx.js

[brainkim]

• implement Channel.race
• implement Channel.merge
• implement Channel.zip
• implement Channel.latest
• add variadic type parameters for these methods
• write tests
• test throwing behavior
• add documentation

Channel.race calls Promise.race on each iteration, Channel.merge merges all async iterators into a single iterator, Channel.zip calls Promise.all on each iteration, and Channel.latest returns an iterator which iterates with the latest values when any individual iterator updates.

[brainkim]

Implemented in commits:
c6acfbb
3e741f0
10cbbc9

TODO: fix the type parameters for these methods.
0e4b770

[brainkim]

TODO: Rename Channel.all to Channel.zip because all implies that all iterators are run to completion when in actuality the iterator returns when any child iterators return.
d8bb3a5

[brainkim]

Released in 1.0