Auto merge of #140999 - hkBst:update-escaper, r=nnethercote

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 rust-lang/rust#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This uses an unpublished version of literal-escaper (https://github.com/rust-lang/literal-escaper/pull/8).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`

Author: bors