introduce derive_eq option

[valsteen]

TODO

• test
• doc

questions:

• derive_hash option ?
• separately mark fields for hash and eq ? - #[sort_by] becomes a wildcard, introduce top-level and per-field #[hash], #[eq] , #[partial_eq], #[ord], #[partial_ord]

[jayvdb]

ping @valsteen , will you be able to finish this?

[valsteen]

@jayvdb hey thanks for giving interest in this project ! I'm getting lost in other non-programming side-projects but here is the struggle I faced when starting this: for consistency, I think all of Eq, PartialEq, Ord, PartialOrd and Hash should have such opt-out option ; but the syntax I started with seems to be a dead end, I feel like I have to break compatibility. Or introduce a guesswork in the parsing so the old ( current ) way still works. Any fresh idea welcome !