[fastpath] Allow compile-time size computation.

[davidtgoldblatt]

In LTO-able contexts, if size is a static constant, then so are szind and usize. But: indirecting through a lookup table obscures this fact from the compiler. Instead, we try to detect if size is static and use a constant-foldable computation if it is.

[interwq]

@davidtgoldblatt we have made plan for this (waiting for buckification to fully rollout). You should sync up with us to avoid conflict like this.

[davidtgoldblatt]

lol, felt a little too easy -- I wondered what the deal was