Start Evaluation

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

Sources/Runtime/Program.swift

@@ -30,4 +30,11 @@ extension ExpressionStatement: EvaluatableParserStatement {
   public func evaluate(execution: ProgramExecution) -> ProgramExecution {
     return execution
   }
+}
+
+// Variables are evaluatable because they can be looked up by identifiers.
+extension Identifier: EvaluatableExpression {
+    public func evaluate(execution: Common.ProgramExecution) -> Result<P4Value> {
+      return execution.scopes.evaluate(identifier: self)
+    }
 }