The scheduler is the core of Yarn, and has tight coupling with the OS-abstracted Scheduler::Fiber class.

Added basic tests - this will be expanded in later changes.

Bug: b/139010488
Change-Id: I562c61d3c4551c4347d9306a3dd87efed06e45a5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34812Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>