py: Add simple way of looking up constants in compiler.

Working towards trying to support compile-time constants (see discussion
in issue #227), this patch allows the compiler to look inside arbitrary
uPy objects at compile time.  The objects to search are given by the
macro MICROPY_EXTRA_CONSTANTS (so they must be constant/ROM objects),
and the constant folding occures on forms base.attr (both base and attr
must be id's).

It works, but it breaks strict CPython compatibility, since the lookup
will succeed even without importing the namespace.

py/mpconfig.h

@@ -150,6 +150,11 @@ typedef double mp_float_t;
 #define MICROPY_EXTRA_BUILTIN_MODULES
 #endif
 
+// Additional constant definitions for the compiler - see compile.c:mp_constants_table.
+#ifndef MICROPY_EXTRA_CONSTANTS
+#define MICROPY_EXTRA_CONSTANTS
+#endif
+
 /*****************************************************************************/
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