Preallocate AVM1 constant pool values

[adrian17]

AFAIK this change shouldn't change any observable behaviors?

This seems reasonable in general (now these objects are actually pooled, and removes a bunch of work from action_push), and is another step towards pre-hashing constant strings like property names in the future.

What positively surprised me, is that apparently AvmString::new called on nearly every ActionPush was a major source of gc-arena GC pressure in some games. For example, in Meat Boy on FF wasm, this change reduced time spent in gc_arena.collect_debt() by 2-3x and - on my PC - improved the game from "slightly choppy most of the time" to "smooth most of the time" :)

Second commit makes avm1::Value Copy, as all its fields are Copy. This generates better code, but I'm not sure if you like the less explicit API.

[Herschel]

Thanks! I agree with changing Value to Copy, there's no good reason not to.