common, codegen: Implement Visitor And Use For CodeGen

Implement a generic visitor for components of a P4 program and use
it to start P4 code generation (according to the behavioral model).

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

Sources/Common/Support.swift

@@ -73,6 +73,14 @@ public func Map<T, U>(input: T, block: (T) -> U) -> U {
   return block(input)
 }
 
+public func Fold<T, A>(input: [T], initial: A, block: (T, A) -> A) -> A {
+  var result = initial
+  for i in input {
+    result = block(i, result)
+  }
+  return result
+}
+
 @freestanding(expression) public macro RequireOkResult<T>(_: Result<T>) -> Bool =
   #externalMacro(module: "Macros", type: "RequireResult")
 @freestanding(expression) public macro RequireErrorResult<T>(