refactor aassign

refactor aassign

move populating stack with return values. Place it
into main loop right after we stored values.

This allow us to delete special if blocks for
hash/array on LHS followed by other hash/array.

Also, clearing HV out of ENTER/LEAVE block is bad -
segfaults in corner cases.

Don't use goto for odd elements case.

store undef on stack for odd case. we can avoid NULL
checks in the loop and use assert like array assignment
does.

use SvSETMAGIC rather than repeate what it means.

for array and hash assignment "while (relem <= SP)" loop
at the end was always empty. Put it into else branch.