Towards a pluggable EventLoop, etc

[olsonjeffery]

In no particular order:

• fixing the ~objects
• creating a BasicEventLoop implementation using standard mutexes and condition variables and converting some of the primitive Scheduler tests to use it
• creating a runtime API for spawning the runtime and/or schedulers with a custom EventLoop
• reviewing PausibleIdleCallback to determine if it's too uv-specific

would be nice to have another event loop (besides libuv) to use to flesh this out. Related to #4419

[toddaaro]

I am very curious what a better design for the PausibleIdleCallback is. It is fairly quirky right now. It might be that a few minor tweaks combined with a rename to "ToggleableIdleBehavior" or something is sufficient.

[olsonjeffery]

from IRC:

<@brson> olsonjeffery: i would have like to make all the tests in sched.rs not rely on libuv
<@brson> olsonjeffery: for a BasicEventLoop i would expose the 'signal' part of the C++ 'lock_and_signal' type,
           which is just a mutex + condition variable, and use that for simulating the libuv async handles