v1.5.1 - change signature of backslashXEncode() and backslashXDecode() slightly

• 1.5.1 (2022-02-25)
  • Breaking Change API of
    backslash_x_encoding.h slightly so that the
    status is returned and the written number of bytes is returned through
    a pointer to an out-parameter.
    • Turns out the more common case is to check the status, and
      ignore the written, so the written pointer can now be nullptr
      if the calling code wishes to ignore it.
    • This is technically a breaking change, but the backslashXEncode()
      and backslashXDecode() methods were just added, but I'm going to
      consider this to be a "quick fix", and increment only the minor
      version number.