Add macro methods for lib-related nodes

[HertzDevil]

Resolves part of #3274. Resolves #5244.

This covers LibDef, CStructOrUnionDef, FunDef, TypeDef, and ExternalVar. Most of those methods should be pretty straightforward, but some warrant a special mention:

• FunDef#real_name, ExternalVar#real_name: These return a StringLiteral rather than a MacroId to minimize the chances that the caller forgets #stringify and produces invalid code. There is no way to distinguish between a node without a real name and a node whose real name is equal to its Crystal name, especially since Crystal::FunDef#real_name is not nilable.
• FunDef#body, #has_body?: The former may return a Nop for both lib funs and top-level funs, so the latter is necessary.
• CStructOrUnionDef#name, TypeDef#name: These return a Path rather than a MacroId to resemble AST nodes outside libs, even though syntactically they cannot be anything other than a single constant.