Issue Description:

It'd be useful to have the range of key.accessibility for declarations.

This would make possible for tools like SwiftLint to validate the order of "modifiers" (private, mutating, final, etc) of declarations.

One ideia would be adding new attributes like key.accessibilityoffset and key.accessibilitylength. Another approach would be adding AccessControl to key.attributes. This second approach seems better to me, as it can also be extended to handle setter accessibility.