[rust] Modularize ruby-prism crate following Ruby's lib/prism structure

[sei40kr]

Summary

• Reorganize ruby-prism crate to mirror lib/prism/ structure
• Extract types into separate modules:
  • node.rs: NodeList, ConstantList, Integer (like node.rb)
  • parse_result/mod.rs: ParseResult, Location (like parse_result.rb)
  • parse_result/comments.rs: Comment, MagicComment (like parse_result/comments.rb)
  • parse_result/diagnostics.rs: Diagnostic (like parse_result/errors.rb)

Verification

This is a pure structural refactoring with no behavioral changes. The following checks were performed to ensure no regressions:

Public API diff (using cargo-public-api)

Removed items from the public API: (none)
Changed items in the public API:   (none)
Added items to the public API:     (none)

Test case names comparison

All 23 test cases preserved with identical names before and after refactoring.

Documentation comments comparison

All doc comments match between before and after (excluding a typo fix: arbitarily → arbitrarily).

Test execution

All 23 tests pass.

Test plan

• cargo test -p ruby-prism passes (23 tests)
• cargo doc -p ruby-prism generates without warnings
• Public API unchanged (verified with cargo-public-api diff)
• Test case names unchanged
• Doc comments unchanged

🤖 Generated with Claude Code

[kddnewton]

Why?

[sei40kr]

@kddnewton
Compared to the Ruby binding, the Rust binding has many missing APIs.
I want to add them, but if I submit PRs in parallel, it will almost certainly cause conflicts.
Furthermore, adding tests and APIs will inevitably make lib.rs huge, making it necessary to split it sooner or later.

[kddnewton]

@sei40kr okay, makes sense. Please make lint pass.

[sei40kr]

@kddnewton My bad, fixed.