Add version-gating for format interactions

[ikeycode]

The original stone format (v0 - unversioned) will be superseded by the first epoch-bump-capable V2 format soon.

In order to facilitate the development of version specific features we need to ensure they are correctly handled by version gating. This means:

• Cargo.toml feature flags for the format variants. Each new version depends on the last feature in TOML and the default remains as v0 for now.
• Handling differences in capabilities or codepaths based on the version number in archive and/or payload headers.

Mapping flags to version numbers to express capabilities seems to be the sanest approach going forwards, allowing writers to raise errors when using mismatched features for the selected versions.