Get rid of RlpDecodingError crutch

[cburgdorf]

What was wrong

We previously introduced a crutch struct RlpDecodingError that we would map to just to then implement From<RlpDecoderError> for PyErr.

How was it fixed

A custom trait

pub trait ToDecodingError<T> {
  fn map_decoder_error(self) -> Result<T, PyErr>;
}

impl<T> ToDecodingError<T> for Result<T, rlp::DecoderError> {
  fn map_decoder_error(self) -> Result<T, PyErr> {
      self.map_err(|val | DecodingError::py_err(val.to_string()))
  }
}

We can then directly call rlp_val.payload_info().map_decoder_error() and goint straight into a DecodingError instead of going through a RlpDecodingError first.

[cburgdorf]

@g-r-a-n-t not trying to drag you into reviews but since I already pinged you I at least wanted to raise attention for this further change that I'm planing to land.