reimplementing Vec<T> and AppendVec<T> from scratch in unsafe rust.
Vec<T> is a growable heap-allocated array with push, pop, insert, remove
and amortized O(1) growth via pointer doubling.
AppendVec<T> is a fixed-capacity, append-only, single-writer multi-reader
vector that uses Release/Acquire atomics to safely publish elements to
concurrent readers without locks, following the same pattern used in
agave's accounts-db.