Adds data-driven tests from json_checker.
Allow a + after the exponent part in number input.
Changes jsonv::detail::string_decode to accept a string_ref.
Issue #15: Correct match_string to jump over '\'s by move forward instead of looking back.

This is more correct *and* easier to read.
Moves json_checker "should fail but don't" files into a special filename pattern.

Potentially think about this in issue #16.