update to literal-escaper 0.0.4 for better API without `unreachable` and faster string parsing This is the replacement for just the part of https://github.com/rust-lang/rust/pull/138163 dealing with the changed API of unescape functionality, since that got moved into its own crate. <del>This uses an unpublished version of literal-escaper (https://github.com/rust-lang/literal-escaper/pull/8).</del> r? `@nnethercote`