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/misc/sys_exc_info.py

@@ -1,4 +1,7 @@
-import sys
+try:
+    import usys as sys
+except ImportError:
+    import sys
 
 try:
     sys.exc_info