Move to generic Chunk type

Right now, the code is locked into `Chunk` being `u64`. This is restrictive for small systems. The `Chunk` type should be made generic. This would be a breaking change.

Open questions: are chunks greater than `u64` worth supporting? If not, should the `Chunk` type be limited to types that are `Into`?

No files, tests, or entry points are named. Start by locating the current `Chunk` type definition and every use that assumes `u64`, then resolve whether larger chunk types are supported or the type is constrained by `Into`. Done means the generic design and its breaking API impact are settled and the affected code remains usable.

Written by the indexing model from the issue text.