Allow stored properties to satisfy property requirements

[kyouko-taiga]

The following currently doesn't compile but it should:

public trait SliceSource {
  type Base
  property contents: PointerToMutable<Base> { let }
}

public type ImmutableSliceSource<Base: Collection>: SliceSource {
  public let contents: PointerToMutable<Base>
  public memberwise init
}

Until this feature is implemented, the workaround is to declare a property that yields the stored value.