consider using `SmallVec` in more places

[y21]

It might be worth in a lot of cases, especially for function calls. I'm guessing that an overwhelming number of function calls in any JS program usually only has up to 3-4 arguments at most. We'll have to experiment and see which numbers make sense (or if it even makes sense at all), but if we can avoid many of these tiny allocations, I think that would already be a good improvement. I know that there's quite a few places in dash where we do something like value.call(this, vec![arg]) (i.e. a one-value vec).