[WEB] take hand-rolled matmul, scalarmul, add, sub
[WEB] take hand-rolled matmul, scalarmul, add, sub

- solid 60fps now, on all three browsers!
- inversion takes about 30% of a frame's time (3ms), so
  that can still be optimized. ~60% of that time is split
  equally between multiply and scalarmul (roughly 1 ms each).

davidyu/Robinson,davidyu/Robinson,davidyu/Robinson