refactor: apply clippy::wrong_self_convention

[shulaoda]

Summary

Three related fixes, all driven by clippy::wrong_self_convention. Run as a single-rule pass: cargo clippy -- -A clippy::all -W clippy::wrong_self_convention.

1. to_* methods take self instead of &self (on Copy types)

The to_* convention is "cheap reference-style conversion", which on Copy types is more natural to express by-value than as &self. No call site needs to change because Copy types are passed implicitly.

• src/comment.rs: CommentStyle::to_str_tuplet
• src/shape.rs: Indent::to_string, Indent::to_string_with_newline, Indent::to_string_inner, Shape::to_string_with_newline

2. Rename to_parsed_config → into_parsed_config

PartialConfig::to_parsed_config already consumes self, so by convention it should be named into_*, not to_*. The only call site (src/config/mod.rs) is updated in the same commit.

3. Rename trait IntoOverflowableItem → AsOverflowableItem

IntoOverflowableItem::into_overflowable_item was misnamed: it takes &self and returns a wrapper that borrows from self (OverflowableItem<'a>). The Into*/into_* convention is reserved for consuming conversions; non-consuming, borrowing conversions should use As*/as_*.

This rename touches:

• The trait, method, and Box<T> blanket impl in src/overflow.rs.
• Both impl_into_overflowable_item_for_* macros and their invocations.
• Generic bounds on rewrite_with_parens, rewrite_with_angle_brackets, rewrite_with_square_brackets, Context::new, and into_overflowable_list in src/overflow.rs.
• Generic bounds on rewrite_array, rewrite_tuple_in_visual_indent_style, and rewrite_tuple in src/expr.rs, plus the corresponding use import.

The helper into_overflowable_list keeps its name (it transforms an Iterator<Item = &T> into Iterator<Item = OverflowableItem<'a>>); only the trait and the per-element method are renamed.

[ytmimi]

Going to close until we decide on which clippy lints we want to enforce in the codebase.