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/string_compare.py

@@ -51,7 +51,10 @@ print("1/" <= "1")
 
 # this tests an internal string that doesn't have a hash with a string
 # that does have a hash, but the lengths of the two strings are different
-import sys
+try:
+    import usys as sys
+except ImportError:
+    import sys
 print(sys.version == 'a long string that has a hash')
 
 # this special string would have a hash of 0 but is incremented to 1