Fault: an optional Cause, kept for the log and never written to the problem #84

A caught exception is lost on the way to a Fault<TFault>: Result.Catch makes the exception the whole error, and the MapError that turns it into the fault an endpoint answers drops it, so a log of the refusal cannot say what failed underneath (decision 52, the roadmap's 2.3.0 section).

Proposal

• An optional Exception? Cause on Fault<TFault>, set through Fault.Of(reason, detail, cause) (and the constructor), carried through WithRule and the composed Fault<T1, T2> / Fault<T1, T2, T3>.
• Equality stays the reason's, the detail's and the rule's: a fault with a cause equals the same fault without one.
• In AspNetCore, FaultProblem<TFault>.Cause and IFaultFeature.Cause (a default member, so a host's own implementation still builds) carry it for the log and CustomizeProblemDetails. It is never written to the problem.
• The docs show MapError(e => Fault.Of(reason, detail, e)) from a Result<T, Exception>.

Done when

• The cause is kept by WithRule, the composed faults and ToHttpResult(), and no problem body names the exception (type or message).
• Equality and hash code ignore it; ToString() prints as 2.2 did.
• Core and AspNetCore READMEs show it as compiled samples; changelog entry.