grep across the repo confirms zero external callers of bintrie.NodeStore,
NewNodeStore, NodeFlushFn, or NodeResolverFn. The arena is purely an
implementation detail of BinaryTrie; unexport the top-level names so
the package's external surface stays confined to BinaryTrie plus the
EIP-7864 helpers (ChunkifyCode, GetBinaryTreeKey*).
Methods on *nodeStore remain capitalized for now — with nodeStore
itself unexported, external code has no way to hold a *nodeStore
pointer, so the methods are effectively internal despite their case.
Method case is a cosmetic follow-up.