SE-0258: Property Wrappers (third review)

[benrimmington]

Suggestions:

1. Could the same name be used for wrapping and unwrapping?
   i.e. init(wrappedValue: T) and var wrappedValue: T.

2. Or could there be an unlabelled initializer to wrap, and a no-parameter subscript to unwrap?
   i.e. init(_: T) and subscript() -> T.

Gold-plating UnboundedRange

I really want to keep empty subscript available as a generalized dereference/unwrap operator idiom. It is incredibly common to have wrapper or accessor types with a primary value (c.f. UnsafePointer.pointee), and the result is a proliferation of .value accessess. I'd like all of those to be spelled [].