implement ThreadPerCore

[jesse99]

This needs to be made useable by rust users. One use case is code that needs to use blocking foreign calls: often this will be done within a task using SingleThread (to avoid blocking other tasks on the thread). But that task may need to spawn sub-tasks, and those should normally be spawned using the default scheduling mode. Unfortunately trying to do so results in task failed at 'thread_per_core scheduling mode unimplemented' so users have to do something lame like using ManualThreads(4).

[eholk]

Don't we get ThreadPerCore scheduling by default?

[brson]

It is the default for the default scheduler, but there is no way to get ThreadPerCore for subsequent schedulers.