Consider `ChunkCursor` maintaining position with pointer

Currently `ChunkCursor` maintains its position in the chunk using a `usize`. Using a pointer instead may be more efficient, although it would take more work to make it sound.

Start by locating the ChunkCursor implementation and reading how its usize position is used throughout the Lox implementation. Determine whether a pointer can preserve soundness and what safety boundaries are required; done means a justified, sound pointer-based position implementation.

Written by the indexing model from the issue text.