Re-use generics infra in SRFI-253 code

[aartaka]

This moves SRFI-253 define-checked to define-method instead of juggling raw lambdas with assertions. Benefits:

• stricter types backed by object system,
• (supposedly) clearer errors,
• extensibility with additional define-method-s,
• and less things to mess up in the implementation, now that it’s all delegated to object system 😅

@egallesio how does it look?