Hashmaps are super expensive and there's no reason not to use a vector - we
already have compact block IDs so we don't benefit from the sparseness. Huge win
for very little effort.
Spotted when profiling FEX. CondJump() in the JIT was almost 4% of our time (?!)
and all because of map slowness. Easy fix.
Difference at 95.0% confidence
-0.0196494 +/- 0.00194956
-3.92827% +/- 0.389753%
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>