compiler, runtime: Begin Runtime Refactor

Ultimately, the goal is to completely separate the compilation from
the runtime to make it possible to have the interpreter/evaluator
be "just another" entity that can perform meaningful work when
given a parsed GP4 program.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Sources/P4Lang/Declarations.swift

@@ -52,7 +52,7 @@ public struct Declaration: P4Type {
     return self
   }
   public var description: String {
-    return "Extern \(self.identifier)"
+    return self.extern ? "Extern " : "" + "\(self.identifier)"
   }
 }
 
@@ -130,3 +130,15 @@ public struct FunctionDeclaration: P4Type, P4DataValue {
     self.body = body
   }
 }
+
+public struct Instantiation {
+  public let name: Identifier
+  public let tipe: P4Type
+  public let arguments: ArgumentList
+
+  public init(named name: Identifier, ofType tipe: P4Type, withArguments arguments: ArgumentList) {
+    self.name = name
+    self.tipe = tipe
+    self.arguments = arguments
+  }
+}