encoding: handle UTF-16 internal limit with fromutf8b (issue5031)...
encoding: handle UTF-16 internal limit with fromutf8b (issue5031)
Default builds of Python have a Unicode type that isn't actually full
Unicode but UTF-16, which encodes non-BMP codepoints to a pair of BMP
codepoints with surrogate escaping. Since our UTF-8b hack escaping
uses a plane that overlaps with the UTF-16 escaping system, this gets
extra complicated. In addition, unichr() for codepoints greater than
U+FFFF may not work either.
This changes the code to reuse getutf8char to walk the byte string, so we
only rely on Python for unpacking our U+DCxx characters.
