process xhv_backreferences early in S_hfreeentries

When deleting a stash, make the algorithm
    GvSTASH($_) = NULL for (@xhv_backreferences);
    delete xhv_backreferences;
    free each stash entry;

Previously the algorithm was

    hide xhv_backreferences as ordinary backref magic;
    free each stash entry:
	this may trigger a sv_del_backref() for each GV being freed
    delete @xhv_backreferences

The new method is:

* more efficient: one scan through @xhv_backreferences rather than lots of
calls to sv_del_backref(), removing elements one by one;

* makes the code simpler; the 'hide xhv_backreferences as backref magic'
hack no longer needs to be done

* removes a bug whereby GVs that had a refcnt > 1 (the usual case) were
left with a GvSTASH pointing to the freed stash; it's now NULL instead. I
couldn't think of a test for this.

There are two drawbacks:

* If the GV gets freed at the same time as the stash, the freeing code
sees the GV with a GVSTASH of NULL rather than still pointing to the
stash.

* As far as I can see, the only difference this currently makes is that
mro_method_changed_in() is no longer called by sv_clear(), but since we're
blowing away the whole stash anyway, method resolution doesn't really
bother us any more.

At some point in the future I might set GvSTASH to %__ANON__ rather than
NULL.

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