manifest: LayerInfos: preallocate the slice

[kolyshkin]

No description provided.

[rhatdan]

LGTM

[mtrmac]

WDYT about make(…, 0, len(…)) + continuing to use append?

I think that achieves the primary optimization goal (not over-allocating, and not calling an allocation function in the loop) without giving up on safety WRT hypothetical future logic changes — at some performance cost, 2 conditional jumps per loop instead of one (sadly, per https://godbolt.org/z/Gbo98a74K , Go can optimize out the range check in [i], but not the capacity check in append).

[kolyshkin]

WDYT about make(…, 0, len(…)) + continuing to use append?

I thought about that, but since we don't do any filtering inside the loop here I went another way.

Changed.

[mtrmac]

Thanks!