RLP derive crate

forked from erlier Apache licenced fastrlp-derive crate, before it changed licence to GPL.

lib.rs:

Derive macro for #[derive(RlpEncodable, RlpDecodable)].

For example of usage see ./tests/rlp.rs.

This library also supports up to 1 #[rlp(default)] in a struct, which is similar to #[serde(default)] with the caveat that we use the Default value if the field deserialization fails, as we don't serialize field names and there is no way to tell if it is present or not.