Introduce type_fingerprint

[soutaro]

This PR introduces #type_fingerprint method that compute a fingerprint of inline RBS type declarations.

• The fingerprint is changed if the inline Ruby code changes it's RBS type declarations -- adding classes, renaming methods, changing method types
• The fingerprint stays same if the inline Ruby code is updated only for it's implementations -- changing the implementation of def node or adding new lines/white spaces to the source code

Using the method, type checkers can detect if the codebase needs re-type-checking after inline Ruby code edits.

[ParadoxV5]

🧠.

Using full nesting arrays over hashes/checksums… intersting.

[soutaro]

The string digest can be computed with the result of to_json. I want a strict change detection, but the digest on the to_json result would work well usually (with lower memory footprint).