Issue Description:

struct S : Comparable { // error: type 'S' does not conform to protocol 'Comparable'
  var x: Int
} 

Maybe it should look something like (straw-man suggestion):

synthesizing Comparable conformances is not supported for structs

Bit of a tangent but... this limitation is just plain weird, given that we support Comparable derivation for enums with associated fields:

enum E : Comparable {
  case e(Int, Int) // Just Works
} 

Should one use single-case enums instead of structgs to get the sweet, sweet conformance synthesis? �