l. Removal of IS_CONSTANT_ARRAY and IS_CONSTANT_INDEX hack
These two #defines disappeared. Instead we have now IS_CONSTANT_AST which
covers also the functionality IS_CONSTANT_ARRAY bid and furthermore the
hack for marking zvals as constant index with IS_CONSTANT_INDEX is now
superfluous and so removed.
Please note that IS_CONSTANT_AST now has the same value than
IS_CONSTANT_ARRAY had.
Because of (from UPGRADING.INTERNALS):