Comparable and FloatingPoint types

[Joe_Groff]

@Letan

EDIT: For a more concrete example of the possible harm, take the following perfectly appropriate implementation of something Equatable :

struct Point : Equatable {
  var x, y: Double
}

However, this struct violates the reflexivity rule in that a == a is not always true. This can lead to some really weird errors down the road. What's also really scary is that any type looking to define equality based on a floating point field is likely to make this mistake by forgetting to handle the nan case.

Steve's proposal would improve this case. Since a struct's default Equatable conformance is derived from the Equatable conformances of its fields, and Double's Equatable conformance would guarantee that a == a, so would the struct's == implementation.