Update v7_stringify() to return length and v7_parse_json() to accept a length

[swisspol]

v7_stringify() should have a way to return the length so that calling strlen() is not needed and v7_parse_json() should accept an optional length parameter (set to ~0 if unused).

This would optimize dealing with non-NUL terminated strings and reduce the number of string copies needed when interfacing v7 with the outside world.