[internal] Different representation for unicode string and byte string in the lambda layer

[bobzhang]

To avoid we accidentialy optimize unicode string operations.
Note since we did validation that is valid utf8, so some string operations like (concat) are still valid, but String.length and String.get should be forbidden

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.