ValueTuple should use readonly methods

[benaadams]

E.g. Equals, GetHashCode etc

/cc @tannergooding

[tannergooding]

More specific instance of https://github.com/dotnet/corefx/issues/36586.

It would probably be good to blanket cover Equals, GetHashCode, and ToString for all mutable structs

[bartonjs]

I've assigned this to Future because #1718 is assigned to Future. It sort of feels like this one should just be closed in favor of that one; but if there's value to having this one separate (and on a different milestone) then I'm curious as to what it is 😄.

[jkotas]

E.g. Equals, GetHashCode etc

Is it going to result into same type of subtle breaking change as what we have hit with Nullable<T> ? #23969 (comment)

[benaadams]

Still valid