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/print_exception.py

@@ -1,10 +1,10 @@
-import sys
-
 try:
     try:
         import uio as io
+        import usys as sys
     except ImportError:
         import io
+        import sys
 except ImportError:
     print("SKIP")
     raise SystemExit