py, compiler: Add basic support for A=const(123).

You can now do:

    X = const(123)
    Y = const(456 + X)

and the compiler will replace X and Y with their values.

See discussion in issue #266 and issue #573.

tests/run-tests

@@ -112,7 +112,7 @@ def main():
     if len(args.files) == 0:
         if pyb is None:
             # run PC tests
-            test_dirs = ('basics', 'float', 'import', 'io', 'misc')
+            test_dirs = ('basics', 'micropython', 'float', 'import', 'io', 'misc')
         else:
             # run pyboard tests
             test_dirs = ('basics', 'float', 'pyb', 'pybnative', 'inlineasm')