Add kind-aware DateTime guards

[safakgur]

Split from #40

Default DateTime.Compare ignores the Kind property. We should:

• Add a new guard, KindSpecified, to require a DateTime argument to have kind that is either Local or Utc, so people can force callers to think about the source of their time values and specify the kinds explicitly.
• Add non-generic overloads to Equal, NotEqual, Min, Max and InRange that accept DateTime arguments and throw if (a.Kind == Unspecified) != (b.Kind == Unspecified) (DateTime already accounts for the time difference when comparing Local values to Utc values).

The second bullet is a breaking change, so I'm adding this to the 2.0 milestone.

[safakgur]

Won't do as the repo is being archived.