topdown/eval: fix 'every' term plugging on save (#4775)

Previously missing plugging could cause unsafe variables in the
PE output.

Now, all terms in the 'every' body should be plugged properly.
The approach taken here is to plug them all, and then fix the
key and val var names of the copied every expression. Those vars
are fresh after the compiler is done with the expression, so
plugging them should never have any effect outside of the rename.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>