Adjust string comparison in jsonpath

Adjust string comparison in jsonpath

We have implemented jsonpath string comparison using default database locale.
However, standard requires us to compare Unicode codepoints.  This commit
implements that, but for performance reasons we still use per-byte comparison
for "==" operator.  Thus, for consistency other comparison operators do per-byte
comparison if Unicode codepoints appear to be equal.

In some edge cases, when same Unicode codepoints have different binary
representations in database encoding, we diverge standard to achieve better
performance of "==" operator.  In future to implement strict standard
conformance, we can do normalization of input JSON strings.

Original patch was written by Nikita Glukhov, rewritten by me.

Reported-by: Markus Winand
Discussion: http://postgr.es/m/8B7FA3B4-328D-43D7-95A8-37B8891B8C78%40winand.at
Author: Nikita Glukhov, Alexander Korotkov
Backpatch-through: 12