constant.pm: Make elements of list consts read-only

constant.pm: Make elements of list consts read-only

Now that Internals::SvREADONLY turns on the PADTMP flag for all the
elements, the read-only flag on the elements themselves will not
actually make the returned elements read-only, because PADTMPs get
copied in all cases where readonliness matters.  What this does is
prevent the original SV from being modified, allowing for more opti-
misations in perl’s internals (e.g., string buffers being stolen from
PADTMPs not marked read-only).

The order of the statements needs to be rearranged, otherwise we end
up setting the flag on a temporary copy of each element due to ‘for’.