chore: Adds variable access to singletons

Author: NeedleInAJayStack

Sources/Haystack/Marker.swift

@@ -6,6 +6,10 @@ public let marker = Marker()
 public struct Marker: Val {
     public static var valType: ValType { .Marker }
 
+    public static var val: Self {
+        return marker
+    }
+    
     public func toZinc() -> String {
         return "M"
     }

Sources/Haystack/NA.swift

@@ -6,6 +6,10 @@ public let na = NA()
 public struct NA: Val {
     public static var valType: ValType { .NA }
 
+    public static var val: Self {
+        return na
+    }
+    
     public func toZinc() -> String {
         return "NA"
     }

Sources/Haystack/Null.swift

@@ -6,6 +6,10 @@ public let null = Null()
 public struct Null: Val {
     public static var valType: ValType { .Null }
 
+    public static var val: Self {
+        return null
+    }
+    
     public func toZinc() -> String {
         return "N"
     }