Iterate static members by type

[DanielSincere]

When generating allStaticMembers, include members only if they are of type Self because a programmer might want a few constants that aren't of the Self type, that shouldn't be iterated through. This will safe guard a common compilation error. Example

@StaticMemberIterable 
struct Foo {
    static let one = Foo()
    static let baseURL = "A string"

    static let allStaticMembers: [Self] = [.one]
}

should expand to

struct Foo {
    static let one = Foo()
    static let baseURL = "A string"

    static let allStaticMembers: [Self] = [.one]
}

Infer type when no matching type

Secondly, if there are no members of type Self, let the compiler infer the type. Example:

@StaticMemberIterable
enum Fixtures {
  static let fixture1: MyType = .init( /*...*/ )
  static let fixture2: MyType = .init( /*...*/ )
}

should expand to

enum Fixtures {
  static let fixture1: MyType = .init( /*...*/ )
  static let fixture2: MyType = .init( /*...*/ )

  static let allStaticMembers = [fixture1, fixture2]
}

Based on feedback from @nevillco

Looks great! For what it's worth, because I was just working on a similar macro & saw this, I think your StaticMemberIterable will include any static member, even if it's not the right type:

@StaticMemberIterable struct Foo {
    static let one = Foo()
    static let two = "A string"

    // Generates (but won't compile)
    static let allValues: [Self] = [.one, .two]
}

I added something (though verbose) to mine to check that the type is Self, here