Problem

Semantic uses a struct of arrays pattern for ScopeTree and other structures.

If we do oxc-project/oxc#4269, then ScopeTree would be purely a collection of IndexVecs. Then we can optimize it.

At present we treat all 6 x IndexVecs as independent. When creating a scope, we push to each vec independently.

This is inefficient in 3 ways:

1. len and capacity are stored 6 times. They only need to be stored once, as they're the same for all 6 vecs.
2. Each time we add a scope, IndexVec::push looks up len for that vec, checks if it's at capacity, and grows if required. These lookups/checks happen 6 times, when they only need to happen once.
3. Looking up multiple pieces of information about a scope will involve multiple bounds checks, when only 1 is required.

Potential solution

Find a crate which optimizes SOA structures to avoid these penalties. Hopefully one exists, but if not, create one.