Add a new private API to optimize Unicode encoders. It uses a small buffer
allocated on the stack and supports overallocation.
Use _PyBytesWriter API for UCS1 (ASCII and Latin1) and UTF-8 encoders. Enable
overallocation for the UTF-8 encoder with error handlers.
unicode_encode_ucs1(): initialize collend to collstart+1 to not check the
current character twice, we already know that it is not ASCII.
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.
Remove an unused early-out test from the critical path for
dict and set lookups.
When the strings already have matching lengths, kinds, and hashes,
there is no additional information gained by checking the first
characters (the probability of a mismatch is already known to
be less than 1 in 2**64).
