compiler, runtime, testing, common: Centralize Execution/Evaluation

Centralize the execution of statements and evaluation of expressions
so that the user can specify a debugging "callback" after every
execution/evaluation.

The callback can be used for myriad things, but it seems likely that
it will be useful for implementing:
1. Testing
2. Debugger

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

Sources/P4Lang/Common.swift

@@ -90,7 +90,7 @@ public struct ParameterList: CustomStringConvertible, Equatable {
 public struct ArgumentList {
   public let arguments: [Argument]
 
-  public init(_ arguments: [Argument]) {
+  public init(_ arguments: [Argument] = []) {
     self.arguments = arguments
   }