Introduces a reference counting mechanism for symbols within the
`FileCache`. When a `SymbolChunkIter` is created, it increments the
reference count for its symbol. When the iterator is dropped, the
reference count is decremented. If a symbol's reference count drops to
zero, all cached data associated with that symbol is evicted from the
cache, freeing up memory. This prevents stale data from being held
indefinitely when no active iterators are using it.
