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/basics/attrtuple1.py

@@ -1,7 +1,10 @@
 # test attrtuple
 # we can't test this type directly so we use sys.implementation object
 
-import sys
+try:
+    import usys as sys
+except ImportError:
+    import sys
 t = sys.implementation
 
 # It can be just a normal tuple on small ports