all: Rename "sys" module to "usys".

This is consistent with the other 'micro' modules and allows implementing
additional features in Python via e.g. micropython-lib's sys.

Note this is a breaking change (not backwards compatible) for ports which
do not enable weak links, as "import sys" must now be replaced with
"import usys".

tests/thread/thread_stacksize1.py

@@ -1,8 +1,10 @@
 # test setting the thread stack size
 #
 # MIT license; Copyright (c) 2016 Damien P. George on behalf of Pycom Ltd
-
-import sys
+try:
+    import usys as sys
+except ImportError:
+    import sys
 import _thread
 
 # different implementations have different minimum sizes