syntax/utf8: avoid a spurious vector reallocation

[lucab]

This reworks Utf8Sequences logic in order to avoid allocating a 0-sized vector and immediately reallocating it for the initial element. Directly create the populated vector instead.

[BurntSushi]

Can you say more about what motivated this change? Is this just something you happened to notice while reviewing the code, or did you notice something in a benchmark?

[lucab]

Hi @BurntSushi and thanks for the prompt reply.
I was looking at the memory usage patterns of rolldown through heaptrack, and this spot showed up as a potentially-spurious temporary allocation. The consumer side is here.
I do not have a specific benchmark for this.

[BurntSushi]

Sounds good. I was mostly just curious. While I agree there is a potentially spurious alloc here (assuming the optimizer doesn't elide it), I doubt it matters relative to all the other work being done. But still, this seems like an improvement. Thank you!