extmod/uasyncio: Add optional implementation of core uasyncio in C.

Implements Task and TaskQueue classes in C, using a pairing-heap data
structure.  Using this reduces RAM use of each Task, and improves overall
performance of the uasyncio scheduler.

extmod/uasyncio/core.py

@@ -4,8 +4,11 @@
 from time import ticks_ms as ticks, ticks_diff, ticks_add
 import sys, select
 
-# Import TaskQueue and Task
-from .task import TaskQueue, Task
+# Import TaskQueue and Task, preferring built-in C code over Python code
+try:
+    from _uasyncio import TaskQueue, Task
+except:
+    from .task import TaskQueue, Task
 
 
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