Update CHANGELOG.md with all changes since v0.12.0:
- Breaking: Path types distinguished at type level
- Breaking: Double slashes in paths are syntax errors
- Breaking: parser/tokenizer modules now private
- Added: FromStr for SyntaxKind
- Added: MIGRATING.md
- Added: fuzz devShell
- Fixed: Leading zero number parsing
- Updated: rowan 0.16, criterion 0.8, expect-test 1.5.1