m2-syn 0.6.0

Relicense m2-syn and m2-syn-macros under MIT; store every comment in the slot where it occurs, so both parsers keep comments in place and Cell::comments() is now an accessor over the new Cell::leading_trivia; add a BOF beginning-of-file marker so parsed files emit their opening, closing, and inter-cell layout byte for byte; recover layout beside comments at delimiters and unclosed groups ending in a line comment; accept x <- y as a parallel assignment target; and retain unfinished string literals in the realtime parsers. SourceFile::new now takes BOF as its first argument. See CHANGELOG.md for migration details.