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.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 9acfc14..2751478 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -109,6 +109,11 @@
 /*****************************************************************************/
 /* Fine control over Python features                                         */
 
+// Whether to enable constant optimisation; id = const(value)
+#ifndef MICROPY_ENABLE_CONST
+#define MICROPY_ENABLE_CONST (1)
+#endif
+
 // Whether to include the garbage collector
 #ifndef MICROPY_ENABLE_GC
 #define MICROPY_ENABLE_GC (0)