sim: implement kernel.heap_stats fake

sim/fakes/kernel.py

+class FakeHeapKindStats:
+    def __init__(self, kind):
+        self.kind = kind
+        self.total_free_bytes = 1337
+        self.total_allocated_bytes = 1337
+        self.largest_free_block = 1337
+
+class FakeHeapStats:
+    general = FakeHeapKindStats('general')
+    dma = FakeHeapKindStats('dma')
+
+def heap_stats():
+    return FakeHeapStats()