docs: Replace ufoo with foo in all docs.

Anywhere a module is mentioned, use its "non-u" name for consistency.

The "import module" vs "import umodule" is something of a FAQ, and this
commit intends to help clear that up.  As a first approximation MicroPython
is Python, and so imports should work the same as Python and use the same
name, to a first approximation.  The u-version of a module is a detail that
can be learned later on, when the user wants to understand more and have
finer control over importing.

Existing Python code should just work, as much as it is possible to do that
within the constraints of embedded systems, and the MicroPython
documentation should match the idiomatic way to write Python code.

With universal weak links for modules (via MICROPY_MODULE_WEAK_LINKS) users
can consistently use "import foo" across all ports (with the exception of
the minimal ports).  And the ability to override/extend via "foo.py"
continues to work well.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

docs/reference/speed_python.rst

@@ -123,7 +123,7 @@ This is a process known as profiling and is covered in textbooks and
 (for standard Python) supported by various software tools. For the type of
 smaller embedded application likely to be running on MicroPython platforms
 the slowest function or method can usually be established by judicious use
-of the timing ``ticks`` group of functions documented in `utime`.
+of the timing ``ticks`` group of functions documented in `time`.
 Code execution time can be measured in ms, us, or CPU cycles.
 
 The following enables any function or method to be timed by adding an
@@ -134,9 +134,9 @@ The following enables any function or method to be timed by adding an
     def timed_function(f, *args, **kwargs):
         myname = str(f).split(' ')[1]
         def new_func(*args, **kwargs):
-            t = utime.ticks_us()
+            t = time.ticks_us()
             result = f(*args, **kwargs)
-            delta = utime.ticks_diff(utime.ticks_us(), t)
+            delta = time.ticks_diff(time.ticks_us(), t)
             print('Function {} Time = {:6.3f}ms'.format(myname, delta/1000))
             return result
         return new_func