C# 7.3: support `==` and `!=` on tuple types

[BillWagner]

Here's the spec: dotnet/csharplang#190

There are several things to discuss for this feature.

My notes here are based on an early draft of the spec. See the reference above for details.

• We need to discuss how lifting works for tuples that have value types compared to tuples with nullable value types: (int, int) == (int?, int)
• Lifting also applies to the tuple itself: (int, int) == (int, int)?
• Conversion operators and deconstruction will not apply for type equality.
• member names are not part of the equality comparison

This will update the references on tuples, but likely won't introduce new articles.

[BillWagner]

The updated proposal is here