Support Self references in traits

[franklinsch]

[franklinsch]

This a subtask of #107 and #338.

[bearbin]

Is this done? We have protection blocks with self, and function calls don't need it. Without adding properties I can't think of any other uses.

[DJRHails]

It's for parameters, where you don't want to support arbitrary traits conforming to it, but specifically the one that is being initialised e.g.

struct trait Asset {
  associatedtype RawType

  // Create the asset by transferring a given amount of asset's contents.
  init(from other: inout Self, amount: RawType)
}