compiler, runtime: Make Default Values Optional

For types, make it optional to return a default value. While default
values for some types are meaningful, they are not meaningful for all
types (e.g., Action, Control, Parser, Parser state, etc.).

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

Sources/P4Lang/Parser.swift

@@ -101,8 +101,8 @@ public class ParserState: P4Type, P4DataValue, Equatable, CustomStringConvertibl
   /// Construct a ParserState
   public init() {}
 
-  public func def() -> any P4DataValue {
-    return ParserState()
+  public func def() -> P4DataValue? {
+    return .none
   }
 }
 
@@ -353,8 +353,8 @@ public struct Parser: P4Type, P4DataValue {
     return "Parser \(self.name) with parameters: \(parameters) and states: \(self.states)"
   }
 
-  public func def() -> any P4DataValue {
-    return Parser(withName: Identifier(name: ""))
+  public func def() -> P4DataValue? {
+    return .none
   }
 }