extmod/utime_mphal: Add generic utime.time_ns() function.

It requires mp_hal_time_ns() to be provided by a port.  This function
allows very accurate absolute timestamps.

Enabled on unix, windows, stm32, esp8266 and esp32.

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/utime_time_ns.py

@@ -0,0 +1,24 @@
+# test utime.time_ns()
+
+try:
+    import utime
+
+    utime.sleep_us
+    utime.time_ns
+except (ImportError, AttributeError):
+    print("SKIP")
+    raise SystemExit
+
+
+t0 = utime.time_ns()
+utime.sleep_us(1000)
+t1 = utime.time_ns()
+
+# Check that time_ns increases.
+print(t0 < t1)
+
+# Check that time_ns counts correctly, but be very lenient with the upper bound (50ms).
+if 950000 < t1 - t0 < 50000000:
+    print(True)
+else:
+    print(t0, t1, t1 - t0)

tests/extmod/utime_time_ns.py.exp

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