refactor: apply clippy::borrow_deref_ref

[shulaoda]

Summary

• Removes redundant &*x patterns where x is already a reference type (&T, &Box<T>, &P<T>, …). Deref-then-reborrow yields the same &T, so the operators add nothing.
• Touches src/expr.rs, src/items.rs, src/macros.rs, src/matches.rs, and src/patterns.rs — 12 sites in total.
• Addresses clippy::borrow_deref_ref, run as a single-rule pass: cargo clippy -- -A clippy::all -W clippy::borrow_deref_ref.

Representative changes:

• rewrite_pair(&*lhs, &*rhs, …) → rewrite_pair(lhs, rhs, …)
• rewrite_unary_prefix(context, &sp_delim, &*rhs, shape) → … , rhs, shape
• rewrite_assign_rhs(context, lhs, &*ty, …) → … , ty, …
• Some(&*block) → Some(block)
• is_short_pattern_inner(context, &*p) → is_short_pattern_inner(context, p)

This is a narrower sibling of clippy::explicit_auto_deref — borrow_deref_ref specifically targets the &*x no-op when x: &T, which auto-deref does not need to mediate at all.

[ytmimi]

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