rust: fix byte_char_slices clippy warnings

rust: fix byte_char_slices clippy warnings

warning: can be more succinctly written as a byte str
   --> src/mime/smtp.rs:762:37
    |
762 |     mime_smtp_find_url_strings(ctx, &[b'\n']);
    |                                     ^^^^^^^^ help: try: `b"\n"`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#byte_char_slices
    = note: `#[warn(clippy::byte_char_slices)]` on by default