Q, R> typedef
Signature for the callback passed to
Q is the type of the message that kicks off the computation.
R is the type of the value returned.
ComputeCallback must be top-level functions or static methods
of classes, not closures or instance methods of objects.
There are limitations on the values that can be sent and received to and
from isolates. These limitations constrain the values of
are possible. See the discussion at