call LEAVE_SCOPE() before POPEVAL()

call LEAVE_SCOPE() before POPEVAL()

All the other POPFOO() types have

    LEAVE_SCOPE(cx->cx_u.cx_blk.blku_old_savestack_ix);

as their first action.

POPEVAL doesn't include this. Instead, each place that does POPEVAL()
currently does a LEAVE_SCOPE() sometime shortly afterwards.
This commit moves all those LEAVE_SCOPE()s to just before each POPEVAL()
to make the behaviour like all the other context types.

This is the logically correct order: process all the savestack items
accumulated during the eval before popping the eval itself.