Add 64-bit offset support for std::io traits

[ncpenke]

To make effective use of larger stable storage size, it would be useful to manipulate stable storage via std::io::Read and std::io::Write traits.

[ncpenke]

This commit fixes this by introducing a new StableIO structure that implements both std::io::Read, and std::io::Write. The existing logic for StableWriter and StableReader is moved under this structure, andStableWriter and StableReader are reimplemented as wrappers around StableIO.

The motivation for this approach was to allow backwards compatibility with existing API, while layering the new functionality/API on top.

This also fixes #334.

If you're interested, I can submit a PR.

[ncpenke]

A further iteration was made by my colleague dmidem@653892c to improve the readability by splitting logic into several files.

In general, happy to make the appropriate cleanup/changes if this functionality seems relevant to cdk-rs.

[lwshang]

Thanks for the suggestion. Please file a PR.

[ncpenke]

Thanks @lwshang created a PR. Happy to incorporate any feedback.