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/run-tests-exp.py

@@ -5,7 +5,7 @@
 # This script is intended to be run by the same interpreter executable
 # which is to be tested, so should use minimal language functionality.
 #
-import sys
+import usys as sys
 import uos as os