rlp_derive

[debris]

this pr introduces 4 new derives:

• RlpEncodable - encodes a structure a list
• RlpDecodable - decodes structure from a list
• RlpEncodableWrapper - encodes structure with a single field, without declaring new list
• RlpDecodableWrapper decodes structure with a single field

Possible improvements:
encoding and decoding enums

[debris]

requires: rust 1.19 which has rust-lang/rfcs#1506

[rphmeier]

@General-Beck ^

@debris What makes me worry about this is that we will know that the structs serialize and deserialize to RLP, but it's a lot harder to know if it's the correct RLP now. This is important for stuff like protocol structures, and existing structures we've written to old databases.

[debris]

@rphmeier I agree, that's why this pr needs careful review. Long term, I believe it's much needed, cause it simplifies our code and reduces amount of boilerplate, the same way serde_derive reduces it for serde.

[General-Beck]

@debris @rphmeier After the release we will switch CI to version 1.19

[rphmeier]

could you toss this at the beginning of the imports with other macro_use guys?

[rphmeier]

There are already tests for the light client which directly serialize the messages with manual RLP streaming (for this reason exactly) so I am not worried about PIP breaking.

Generally, having a nice suite import/regression tests (parity-import-tests repo) will protect against regressions here as well.