Signature of a map/reduce computation.
val compute : ThreadPoolExecutor.t ->
input Stream.t -> output -> output
Iterates over the passed stream using pool threads to execute
map operations over the various input values, and applies
combine operation over values having identical keys as soon as
they are available.
Then, folds over key/value couple through the reduce operation,
using the third parameter as the inital value.
Raises Runtime.Raise
if a map, reduce, combine
operation, or comparison raises and exception.