Flatten outputs for chained boolean operations where possible

[rohanpm]

Add special-case handling to flatten the data structure when chaining
multiple AND/OR together; for example, make f_and(f_and(x, y), z)
work approximately in the same way as f_and(x, y, z).

The goal here is to avoid a stack overflow when using a pattern which
can chain a large number of futures. Previously, an overflow could start
to happen if the chain was a few hundred futures deep, since each
future's completion callback would set the result on the next future
which would invoke another completion callback and so on throughout the
chain.

Fixes #320