Missing semaphore functions

[schteve]

It looks like these two functions are missing from the shim layer. We might want to add them someday. There are also FromISR variants for them.

• xSemaphoreGetMutexHolder()
• uxSemaphoreGetCount()

It's not much help just having the functions in the shim layer - we'd also want to add Rust code to make use of it. Both are probably straightforward as we already have Mutex types. Semaphore doesn't have a Counting-specific type (should it?), but GetCount is defined on both counting and binary semaphores anyway.