Streamline default EqualityComparer and Comparer for Enums

[jkotas]

This borrows the implementation strategy for these from CoreRT. It makes it both simpler (fewer types and lines of code) and faster in some cases since we always use the exact right underlying type.

E.g. The following micro-benchmark is 25% faster with this change:

enum MyEnum : byte { x, y };

var comparer = Comparer<MyEnum>.Default;

for (int i = 0; i < 100000000; i++)
{
    comparer.Compare(MyEnum.x, MyEnum.y);
    comparer.Compare(MyEnum.y, MyEnum.x);
}

[jkotas]

cc @marek-safar

[Suchiman]

Just a nitpick but where T : Enum does not preclude classes so it would be possible to pass boxed enums, e.g. EnumCompareTo<Enum>(null, null). To avoid this, where T : struct, Enum should be used.

[jkotas]

@Suchiman Thanks for taking a look

[stephentoub]

Simpler and faster is a nice combination :)

[marek-safar]

Fabulous!

Is breaking serialization compatibility between 2.x and master concern?

[stephentoub]

This shouldn't break compatibility because of the SetType that's used. @ViktorHofer can confirm, but I believe we have tests for this specifically.

[marek-safar]

Right, but how do you hit it if the type to deserialize is e.g. UInt64EnumComparer ?

[ViktorHofer]

A serialization payload will never contain the type UInt64EnumComparer because when such an object is serialized, it masks itself as an ObjectComparer<>. That also implies that if serialization is involved you won't benefit from these gains.