lib: Improve Unicode handling for string schema keywords
lib: Improve Unicode handling for string schema keywords

The ‘maxLength’ and ‘minLength’ keywords handle string lengths. On
checking RFC 4627 (JSON), it’s confirmed that JSON strings calculate
their lengths in terms of Unicode characters.

Expand the generation functions for maxLength and minLength to also
generate Unicode strings and remove some TODO comments.

pwithnall/walbottle