Custom main and global executors work hasn't passed Swift Evolution yet,
so we need to avoid leaking it as API until it does.
To that end, underscore all the things.
rdar://151147606
EventableExecutor is being removed, for now, but hopefully will
return in some form in the future.
The `asSchedulable` implementation needs to change for reasons of
ABI stability.
rdar://141348916
Remove `supportsScheduling` in favour of a type-based approach.
Update the storage for `ClockTraits` to `UInt32`.
Adjust ordering of executors for `currentExecutor`.
rdar://141348916
Added an `-executor-factory` argument to the compiler to let you safely
specify the executors you wish to use (by naming a type that returns
them).
Also added some tests of the new functionality.
rdar://141348916
