When implementing a trait for an object or when re-opening an object,
the compiler required that you specify the type parameter names of the
object. For example, to implement the Equal trait for Triple you would
write:

    impl Equal for Triple!(A, B, C) {
      ...
    }

The original idea of this syntax was to make it more clear that type
parameters used in the body originated from the object, not the trait.

Over time, I realised that this doesn't actually help and in fact makes
the compiler's life more difficult in a bunch of places. In this commit,
we remove the need for specifying the type parameters, allowing you to
write the following instead:

    impl Equal for Triple {
      ...
    }