Go: make `EncodeTo()` generation consistent with `DecodeFrom()`

[2opremio]

• Method receiver names in DecodeFrom (u for unions, e for enums etc ...) are consistent, while not in EncodeTo
• DecodeFrom provides context when returning errors (indicating the name of the type), while EncodeFrom doesn't
• DecodeFrom uses the decoderFrom interface while EncodeTo uses the xdrType interface.