wasm: Improve planner to reuse local variables

Previously the planner would never reuse local variables across
blocks. This becomes a problem for large query sets (e.g., due to
partial eval) or large rule sets (e.g., due to disjunction). This
change updates the planner to reuse local variables across
blocks. Anecdotally this became a problem with 10K rules that each
performed a 3-tuple check.

Also, restore the target variable after planning rules. Although this
does not matter the planner state should be fully restored when
planRules returns. Perhaps it's time to refactor so that planner state
does not have to be mutated so much.

Signed-off-by: Torin Sandall <torinsandall@gmail.com>