GeezerVison

…d bytearray

_Py_bytes_lower() and _Py_bytes_upper() now convert strings of at least
one machine word (8 bytes on 64-bit platforms, 4 on 32-bit) a word at a
time with integer arithmetic.  When the length is not a multiple of the
word size, the last word overlaps the one before it.  Shorter strings
still take the existing table loop.

str.lower(), str.upper() and str.casefold() use these functions for
ASCII strings, as do lower(), upper() and capitalize() on bytes and
bytearray.

Count words instead of bytes, index them through an array-of-words
type, and compute the two per-call constants before the loop.  MSVC
only vectorizes loops whose counter steps by one, and with the
multiplication by "first" inside the loop it vectorizes that with an
AVX-512 instruction and runs the vector loop only on AVX-512 CPUs.

The unsigned word count avoids an idiv in size-optimized (PGO) code,
and converting the overlapping last word first keeps the loop as
short as before for compilers that don't vectorize it.

Counting words makes clang use post-indexed loads and stores on
AArch64, which are slower on Apple cores, so step by bytes as before
everywhere except under MSVC, which only vectorizes a loop that counts
by one.  Both loops get the constants from outside the loop (otherwise
MSVC vectorizes the multiplication with an AVX-512 instruction), and
the overlapping last word is converted after the loop again.