[swift-evolution] [Pitch] Rename `x.dynamicType` to `x.Self`

[Joe Groff]

It's been pitched before, but I don't think we've had a dedicated thread to this idea. Erica has proposed making `Self` generally available within methods in types to refer to the dynamic type of the current receiver. One could think of `Self` as a special associated type member that exists in every type for this purpose. This also happens to be what you get when ask for the `dynamicType` member of a value. We could unify these concepts and get rid of the clunky `dynamicType` keyword, replacing it with `x.Self`.

There's another benefit to this syntax change. Looking to the future, one of the many features Doug pitched in his generics manifesto was to generalize protocol existentials, lifting our current restrictions on protocols "with Self or associated types" and allowing them to be used as dynamic types in addition to static generic constraints. Once you do this, you often want to "open" the type of the existential, so that you can refer to its Self and associated types in the types of other values. I think a natural way would be to let you directly use Self and associated type members of existentials as types themselves, for example:

	let a: Equatable = /*...*/
	let b: Equatable = /*...*/

	// This is not allowed, since Equatable requires two values with the same static type, but
	// a and b may have different dynamic types.
	a == b 

	// However, we can dynamically cast one to the other's dynamic type:
	if let bAsA = b as? a.Self {
		return a == bAsA
	}

	let x: RangeReplaceableCollection = /*...*/
	let y: Collection = /*...*/

	// If y has the same dynamic Element type as x, append it to x
	var z: x.Self = x
	if let yAsX = y as? Any<Collection where Element == x.Element> {
		z.append(yAsX)
	}

`x.Self` then becomes just the first step in this direction.

-Joe