py/modmicropython: Add heap_locked function to test state of heap.

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.

tests/micropython/heap_lock.py

@@ -5,6 +5,7 @@ import micropython
 l = []
 l2 = list(range(100))
 
+micropython.heap_lock()
 micropython.heap_lock()
 
 # general allocation on the heap
@@ -19,6 +20,14 @@ try:
 except MemoryError:
     print('MemoryError')
 
+print(micropython.heap_unlock())
+
+# Should still fail
+try:
+    print([])
+except MemoryError:
+    print('MemoryError')
+
 micropython.heap_unlock()
 
 # check that allocation works after an unlock

tests/micropython/heap_lock.py.exp

@@ -1,3 +1,5 @@
 MemoryError
 MemoryError
+1
+MemoryError
 []

tests/micropython/heap_locked.py

@@ -0,0 +1,12 @@
+# test micropython.heap_locked()
+
+import micropython
+
+if not hasattr(micropython, "heap_locked"):
+    print("SKIP")
+    raise SystemExit
+
+micropython.heap_lock()
+print(micropython.heap_locked())
+micropython.heap_unlock()
+print(micropython.heap_locked())

tests/micropython/heap_locked.py.exp

@@ -0,0 +1,2 @@
+1
+0