The utf-16* and utf-32* encoders no longer allow surrogate code points
(U+D800-U+DFFF) to be encoded.
The utf-32* decoders no longer decode byte sequences that correspond to
surrogate code points.
The surrogatepass error handler now works with the utf-16* and utf-32* codecs.

Based on patches by Victor Stinner and Kang-Hao (Kenny) Lu.

#8271: the utf-8 decoder now outputs the correct number of U+FFFD  characters when used with the "replace" error handler on invalid utf-8 sequences.  Patch by Serhiy Storchaka, tests by Ezio Melotti.

endianess detection and handling.

Issue #15144: Fix possible integer overflow when handling pointers as integer values, by using Py_uintptr_t instead of size_t.
Patch by Serhiy Storchaka.

Patch by Serhiy Storchaka.

Patch by Serhiy Storchaka.

The main bottleneck was the PyUnicode_READ() macro.

This almost catches up with pre-PEP 393 performance, when decoding needed
only one pass.