The via() and on() algorithms should take a scheduler

[lewissbaker]

The via() and on() algorithms currently take a sender that represents the execution context transition rather than taking the scheduler. This means the caller needs to explicitly call schedule(s) to get the sender to use.

These algorithms should be changed to take a scheduler instead.

We may need to add two new algorithms that compose senders in the way that these algorithms currently do to replace their utility.

[lewissbaker]

PR #44 changes on() to take a scheduler. Still need to tackle via().

[lewissbaker]

PR #47 changes typed_via() to take a scheduler.

[janondrusek]

@lewissbaker is this OK to close? Seems like via() was addressed in PR #273.