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/float/float2int_intbig.py

@@ -2,10 +2,10 @@
 
 try:
     import ustruct as struct
+    import usys as sys
 except:
     import struct
-
-import sys
+    import sys
 
 maxsize_bits = 0
 maxsize = sys.maxsize