decoder, encoder: always use "readable-stream"

For the Transform class, always use the version from
"readable-stream". This way, we are confident that we're
using the correct API for implementing the stream, etc.
decoder: never output NUL bytes in the ID3 strings

Use a combination of `strlen()` and `mpg123_strlen()` in
order to guarantee that no trailing NUL bytes will be
included in the String.