Better syntax for wrapping other comparers

[StephenCleary]

It is sometimes desirable to wrap other comparers with a Nito.Comparers comparer that just forwards to the underlying comparer. This brings the benefit of not throwing when GetHashCode(null) is called (see #13). We could make this wrapping easier.

Counterpoint: this wrapping makes it easy to treat an IComparer<T> as though it were an IEqualityComparer<T> (since the wrapper is an IFullComparer<T>). Currently, this would cause a runtime error if GetHashCode is used without being implemented (see https://github.com/StephenCleary/Comparers/wiki/Pitfalls). By making the wrapping easier, this pitfall is more exposed. However, it probably will remain a rare occurrence.

Possible syntax options:

• Simple extension method: StringComparer.Ordinal.NormalizeNullHandling(). Not too wild about the naming.
• Make the delegate in SelectBy/EquateBy optional (default to the identity function): ComparerBuilder.For<string>().CompareBy(StringComparer.Ordinal). Makes it clear that this is a Nito comparer, but seems pretty verbose.

[StephenCleary]

v6 will have a WithStandardNullHandling extension method in the Nito.Comparers.Fixes namespace.

https://github.com/StephenCleary/Comparers/blob/f706547c923d296e9f1f0651c0940c68f107a1de/test/UnitTests/FixComparers.cs#L12-L23