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".

docs/library/index.rst

@@ -77,7 +77,6 @@ it will fallback to loading the built-in ``ujson`` module.
    cmath.rst
    gc.rst
    math.rst
-   sys.rst
    uarray.rst
    uasyncio.rst
    ubinascii.rst
@@ -93,6 +92,7 @@ it will fallback to loading the built-in ``ujson`` module.
    usocket.rst
    ussl.rst
    ustruct.rst
+   usys.rst
    utime.rst
    uzlib.rst
    _thread.rst

docs/library/usys.rst

@@ -1,7 +1,7 @@
-:mod:`sys` -- system specific functions
-=======================================
+:mod:`usys` -- system specific functions
+========================================
 
-.. module:: sys
+.. module:: usys
    :synopsis: system specific functions
 
 |see_cpython_module| :mod:`python:sys`.
@@ -28,10 +28,10 @@ Functions
       This function is a MicroPython extension intended to provide similar
       functionality to the :mod:`atexit` module in CPython.
 
-.. function:: print_exception(exc, file=sys.stdout, /)
+.. function:: print_exception(exc, file=usys.stdout, /)
 
    Print exception with a traceback to a file-like object *file* (or
-   `sys.stdout` by default).
+   `usys.stdout` by default).
 
    .. admonition:: Difference to CPython
       :class: attention
@@ -84,7 +84,7 @@ Constants
    value directly, but instead count number of bits in it::
 
     bits = 0
-    v = sys.maxsize
+    v = usys.maxsize
     while v:
         bits += 1
         v >>= 1
@@ -113,7 +113,7 @@ Constants
    is an identifier of a board, e.g. ``"pyboard"`` for the original MicroPython
    reference board. It thus can be used to distinguish one board from another.
    If you need to check whether your program runs on MicroPython (vs other
-   Python implementation), use `sys.implementation` instead.
+   Python implementation), use `usys.implementation` instead.
 
 .. data:: stderr