Unescape quoted-pairs inside quoted-strings

[gaoflow]

parse_content_disposition(r#"attachment; filename="a\"b.txt""#) returns the filename as a\"b.txt instead of a"b.txt — the quotes around a parameter value get stripped but the quoted-pairs inside them are never undone. Two neighbouring spots have the same gap: Display for SingleInfo/GroupInfo escapes " in a display name but not \, so the name Foo\Bar renders as "Foo\Bar" <x@y.com> and addrparse reads that straight back as FooBar — the crate emits something its own parser silently corrupts; and the comment state ends on an escaped \), so x@y.com (a\)b) parses to the address x@y.com b). The quoted display-name reader is the one place that already gets this right, which is why the round-trip loses data rather than erroring.

In display names I kept RFC 5322 §3.2.1 quoted-pair in full (backslash before anything, obs-qp included) since that is what the quoted-name reader already does. In MIME parameter values I only undo \" and \\ and leave every other \x alone, so filename="C:\dev\go\foo.txt" still comes out intact — that is where Python's email and Go's mime both draw the line, and I checked each row of the change against both (the only case they disagree on is \;, which I left untouched). A backslash outside quotes stays literal either way; the existing parse_backslashes test fails if that gets widened.

cargo test --all goes from 54 to 56 unit tests plus 25 doctests, all green; cargo fmt -- --check is clean and cargo clippy shows no new warnings.