make sv_clear() iterate over AVs

make sv_clear() iterate over AVs

In sv_clear(), rather than calling av_undef(), iterate over the AV's
elements. This is the first stage in making sv_clear() non-recursive,
and thus non-stack-blowing when freeing deeply nested structures.

Since we no longer have the stack to maintain the chain of AVs currently
being iterated over, we instead store a pointer to the previous AV in the
AvARRAY[AvMAX] slot of the currently-being-iterated AV. Since our first
action is to pop the first SV, that slot is guaranteed to be free, and
(in theory) nothing should be messing with the AV while we iterate over
its elements, so that slot should remain undisturbed.