chore: Use HashMap instead of BTreeMap for storing fields by id in StructType

[amogh-jahagirdar]

This change updates the id_lookup table in StructType to be a HashMap implementation instead of BTreeMap. Since there won't be a need to do min/max or any kind of ordering, a BTreeMap shouldn't be necessary, and lookups for a field based on id can just be done in O(1).

cc: @JanKaul @Xuanwo @Fokko

[Xuanwo]

LGTM, thanks!

[liurenjie1024]

LGTM, thanks

[Fokko]

I would also lean towards a HashMap. Probably this is also more memory efficient (fewer pointers than with a BTree).

Don't worry, we're going to use the BTree when we're digging into SequenceNumbers

[ZENOTME]

LGTM! Thanks!

[amogh-jahagirdar]

Thanks for all the reviews @Fokko @Xuanwo @ZENOTME @liurenjie1024. Since there's a few approvals and it's small change, I'll be merging.