See epic for context.

The goal here is to get efficiency on par with the underlying C APIs.

We already have resultView(), but this has two problems:

• holds the data in an internal buffer from which it needs to be copied by the user to persistent storage before the QCH object is destroyed
• in particular, and this is the second point, it doesn't work for the static hash() short-cut
  • the internal buffer doesn't survive the function call, so we need to return by QByteArray

The idea of -Into() is to bypass the internal buffer and use a user-provided one.

Acceptance criteria:

• hash() and result() have -Into() versions that allow the user to pass a destination buffer
  • as QByteArray*
  • as QSpan<std::byte> / QMutableByteArrayView (both would need to be written first)
• hashInto() does not copy the result
  • doesn't use Private::result at all