Complete Types::Block hash semantics

[OskarEichler]

Summary

RBS::Types::Block implements value equality with ==, but does not alias eql? or define a corresponding hash. Equal block types therefore behave as different Hash/Set keys, unlike the surrounding immutable type objects.

Add eql? and compute the hash from the same type, required flag, and self type used by equality.

Verification

• baseline model reproduces the Hash-key miss; focused and cumulative candidates pass
• type tests: 3 tests / 44 assertions
• all 109 runtime files compile on Ruby 4.0.6
• focused candidate syntax, RuboCop, and diff checks pass

Source-only change; no test file is included.