runtime: Refactor Expression/Statement Evaluators

Allow the user to customize the evaluation/execution of expressions/
statements with something that implements functions that perform
those tasks. This additional functionality will make it possible for
the "classic" evaluator not to waste time checking for the presence
of interlopers and give implementers additional customization opportunities.

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

Sources/P4Runtime/Runtime.swift

@@ -59,7 +59,7 @@ public struct ParserRuntime: CustomStringConvertible {
   {
     let pe =
       if let initial = initialValues {
-        ProgramExecution(withGlobalValues: initial)
+        ProgramExecution().setGlobalValues(initial)
       } else {
         ProgramExecution()
       }