Add a bichain function

[Avaq]

bichain :: (a -> Future x y) -> (b -> Future x y) -> Future a b -> Future x y

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Currently the only way to bichain is like this:

compose (chain (x => x)) (coalesce (f) (g))

It's not straightforward and one might be inclined to do something like this instead:

compose (chainRej (f)) (chain (g))

However, this solution behaves different from how bichain would, in that if g returns a rejected Future, its rejection is handled by f.