Consider adding helpful implementation to `SphereFile`

[cdata]

Per discussion here, we should consider if it would be helpful for SphereFile to have it's own helper implementation. Maybe it is useful as a pointer to ancestry for future writes? Let's discuss!

[jsantell]

Inlining incase resolved comments in PR vanish after squashing:

Might be notable that NsSphereFile is only used for lazily reading data, whereas writing (and saving) data is done at the NsSphereFs level e.g. there's no ns_sphere_file_write(). No opinions on changes at the moment, but two possibilities depending on future enhancements could be either having NsSphereFile have more of its own "methods", or being more of a lazy-readonly byte reader.