Add a pony primitive that exposes scheduler information

[SeanTAllen]

Previously, when users wanted to optimize the parallelizing work based on the maximum number of actors that can be run at a single time, we were pointing them to using the private runtime method @ponyint_sched_cores().

This was problematic for two reasons:

1. We were pointing users at an internal implementation method that we hadn't promised not to change.
2. We were requiring users to muck about with FFI for something that has become somewhat common for users wanting to maximize performance want to know.

We've now exposed some information about scheduler threads via a new package runtime_info thereby resolving both of the problems mentioned above.

Pony users should expect to see additional information added to the runtime_info package moving forward.

Anyone who was using the internal ponyint_sched_cores() FFI call will need to update to using runtime_info as ponyint_sched_cores() has been removed.

Closes #3985

[SeanTAllen]

When this is merged, it should be noted in last week in pony that RFC 71 has been implemented.