This commit adds micropython.heap_locked() which returns the current
lock-depth of the heap, and can be used by Python code to check if the heap
is locked or not. This new function is configured via
MICROPY_PY_MICROPYTHON_HEAP_LOCKED and is disabled by default.
This commit also changes the return value of micropython.heap_unlock() so
it returns the current lock-depth as well.
Behaviour was changed from stack to queue in
8977c7eb58, and this updates variable names
to match. Also updates other references (docs, error messages).
Otherwise there is really nothing that can be done, it can't be unlocked by
the user because there is no way to allocate memory to execute the unlock.
See issue #4205 and #4209.
