Remove Combinator/Resolver in favor of functions

This commit removes the Combinator and Resolver classes altogether.
The relevant functionality is now concentrated entirely in the
analogous functions:

- all()
- some()
- any()
- first()
- map()
- filter()
- resolve()

Additionally, all Promisor implementations now *require* a Reactor
upon instantiation. Previously this parameter could be null and the
static singleton reactor instance would be used automatically.
Require Reactor when instantiating Promisor implementations
Update changelog