Document Series' methods inherited from Promise
Refactor `timed` to work as expected with thenables and add documentation
Fixes and documentation for `concurrent`

* `parallel()` was no longer parallel after the `Series` refactor
* `sequence()` and `parallel()` should return a `Series` instance, since they return arrays
Tweak `fn` and add documentation

* `compose()` should assimilate thenables for consistency with other `fn` methods