Issue Description:

I have found numerous instances in which I would like to provide partial protocol conformance in a superclass, while leaving some of the protocol requirements unfulfilled. Traditionally this is called a partially abstract class. While Swift's protocols act as traditional purely abstract classes/structs, it would be helpful to have partially abstract classes/structs which leave some requirements for descendants to fulfill.

I would expect that abstract classes and abstract structs could not be directly initialized and that the init methods could only be called from subclasses' init methods.

I would like to use an abstract keyword like this:

internal
protocol
Plant
{
    
    var plant_genus   : String
    { get }
    var plant_species : String
    { get }
    
}

internal
abstract
struct
Astragalus
: Plant
{
    
    internal var plant_genus : String
    { return "Astragalus" }
    
}

internal
strict
AstragalusAlpinus
: Astragalus
{
    
     internal var plant_species : String
     { return "Astragalus alpinus" }
     
}