gh-139871: Optimize bytearray construction with encoding

[cmaloney]

When a str is encoded in bytearray.__init__ the encoder tends to create a new unique bytes object. Rather than allocate new memory and copy the bytes use the already created bytes object as bytearray backing. The bigger the str the bigger the saving.

Mean +- std dev: [main_encoding] 497 us +- 9 us -> [encoding] 14.2 us +- 0.3 us: 34.97x faster

import pyperf

runner = pyperf.Runner()

runner.timeit(
    name="encode",
    setup="a = 'a' * 1_000_000",
    stmt="bytearray(a, encoding='utf8')")

• Issue: Add .take_bytes([n]) a zero-copy path from bytearray to bytes #139871

[cmaloney]

cc: @vstinner this construction form doesn't appear a lot in the CPython codebase but does exist in other codebases.

This one I think is a safe subset from #141862; hope to revisit that eventually but it's definitely many-step to get working just right.

[vstinner]

LGTM. It seems to be safe to pick the bytes object in this case.