v5.11.0 Swift 6 Compatibility

This release conforms various types to Sendable and eliminates all Swift 6 language mode compilation errors.

To the best of my knowledge, adding Sendable conformances isn't a breaking change so this is versioned as v5.11.0. If you find that this release breaks your build, please open an issue and I can release make a v5.11.1 that reverts the changes, then re-release them as v6.0.0.