Expose functions that return `str` instead of `bytes`

It would be nice to get canonical json as str, as a drop-in replacement for `json.dumps`. Right now using `encode_canonical_json` for that means going back and forth between str and bytes which is wasteful.
Thanks!

Start by locating the encode_canonical_json entry point and compare its bytes result with json.dumps, which returns str. Add the requested str-returning API and verify that canonical JSON output is available without a str/bytes conversion; the issue does not name specific files or tests.

Written by the indexing model from the issue text.