TODO

* tests for both std and eastl
* remove the return_type solution in types.h
* support for auto parameter type deduction for .then handlers
* benchmark vs continuables (both synthetic tests and real-world)
* measure how much partial application would cost (+ unpacking tuples)
* measure how much the void support costs
* static assert that verifies that callbacks return mc::result (and other cases)
* guards against multiple invocation of promise
* future<T...> instead of future<tuple<T...>> for compositions. Note: not high prio; future<tuple<T...>> should only be used as intermediate steps; not referenced directly