py/modmicropython: Add micropython.const, alias for identity function.

Having a micropython.const identity function, and writing "from micropython
import const" at the start of scripts that use the const feature, allows to
write scripts which are compatible with CPython, and with uPy builds that
don't include const optimisation.

This patch adds such a function and updates the tests to do the import.

tests/micropython/const.py

@@ -1,5 +1,7 @@
 # test constant optimisation
 
+from micropython import const
+
 X = const(123)
 Y = const(X + 456)
 

tests/micropython/const2.py

@@ -1,5 +1,7 @@
 # check that consts are not replaced in anything except standalone identifiers
 
+from micropython import const
+
 X = const(1)
 Y = const(2)
 Z = const(3)

tests/micropython/const_error.py

@@ -1,5 +1,7 @@
 # make sure syntax error works correctly for bad const definition
 
+from micropython import const
+
 def test_syntax(code):
     try:
         exec(code)