bpo-37587: Make json.loads faster for long strings (GH-14752)

When scanning the string, most characters are valid, so
checking for invalid characters first means never needing
to check the value of strict on valid strings, and only
needing to check it on invalid characters when doing
non-strict parsing of invalid strings.

This provides a measurable reduction in per-character
processing time (~11% in the pre-merge patch testing).