Improved `Partial{Eq,Ord}` derive

[mcarton]

Provide derivative({PartialEq, Eq, PartialOrd, Ord}) with:

• Eq:
  • custom bound
• PartialEq:
  • custom bound
  • ignore on a field
  • compare_with on a field
• Ord:
  • custom bound
  • ignore on a field
  • compare_with on a field
• PartialOrd:
  • custom bound
  • ignore on a field
  • compare_with on a field

enum are tricky¹ as there is no stable way to compare discriminants yet. See rust-lang/rfcs#1696.

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1: not for Eq.

[mcarton]

Eq is done. I'm going to wait a while on dtolnay/syn#27 for the others.

[elidupree]

What's the current status of this? I have a project where it would be significantly helpful if I could derive Ord with custom bounds (to work around rust-lang/rust#26925 ). I'd submit a PR, but unfortunately the proc-macro code is still fairly opaque to me and I don't want to do it wrong.