Currently, the only way to concatenate an integer to a bytes object is by converting
the integer to bytes with a function call before concatenating. And there is no way to
make a mutable bytes object without a function call. And bytearray function calls are 9-10x slower than b-strings.

I propose an array-type string like the, or for the bytearray. It would work as a
mutable b-string, as

foo = a"\x00\x01\x02abcÿ"       # a-string, a mutable bytes object.
foo[0] = 123                              # Item assignment
foo+= 255                                 # Works the same as bytesvariable+=b"ÿ"
foo+= a"\x255\x00"                  # Concatenation with itself
foo+= b"\x255\x00"                  # Cross compatibility with bytes objects.

This would be processed the same as, or would be the bytearray,

>>> type(a"\x00\x01\x02abcÿ")
<class 'bytearray'>