Explicit as_str (no deref), add no allocation methods (#8826)

## Summary

This PR is a follow-up to the AST refactor which does the following:
- Remove `Deref` implementation on `StringLiteralValue` and use explicit
`as_str` calls instead. The `Deref` implementation would implicitly
perform allocations in case of implicitly concatenated strings. This is
to make sure the allocation is explicit.
- Now, certain methods can be implemented to do zero allocations which
have been implemented in this PR. They are:
    - `is_empty`
    - `len`
    - `chars`
    - Custom `PartialEq` implementation to compare each character

## Test Plan

Run the linter test suite and make sure all tests pass.

crates/ruff_python_ast/src/all.rs

@@ -24,7 +24,7 @@ where
     fn add_to_names<'a>(elts: &'a [Expr], names: &mut Vec<&'a str>, flags: &mut DunderAllFlags) {
         for elt in elts {
             if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = elt {
-                names.push(value);
+                names.push(value.as_str());
             } else {
                 *flags |= DunderAllFlags::INVALID_OBJECT;
             }