Use dedicated AST nodes on MemberKind (#6374)

## Summary

This PR leverages the unified function definition node to add precise
AST node types to `MemberKind`, which is used to power our docstring
definition tracking (e.g., classes and functions, whether they're
methods or functions or nested functions and so on, whether they have a
docstring, etc.). It was painful to do this in the past because the
function variants needed to support a union anyway, but storing precise
nodes removes like a dozen panics.

No behavior changes -- purely a refactor.

## Test Plan

`cargo test`

crates/ruff_python_ast/src/lib.rs

@@ -3,7 +3,6 @@ use std::path::Path;
 
 pub mod all;
 pub mod call_path;
-pub mod cast;
 pub mod comparable;
 pub mod docstrings;
 pub mod hashable;