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>

Tests/p4rseTests/CodeGenTests/CodeGen.swift

@@ -0,0 +1,98 @@
+// p4rse, Copyright 2026, Will Hawkins
+//
+// This file is part of p4rse.
+//
+// This file is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+import Common
+import Foundation
+import Macros
+import P4Compiler
+import P4Lang
+import P4Runtime
+import SwiftTreeSitter
+import Testing
+import TreeSitter
+import TreeSitterP4
+
+@testable import P4Runtime
+
+func shrink(_ from: String) -> String {
+  return from.replacing(Regex(#/\n[\s]+/#), with: "").replacing("\n", with: "")
+}
+
+@Test func test_codegen_parser_state_direct_transition() async throws {
+  let simple_parser_declaration = """
+    parser main_parser() {
+       state start {
+           true;
+           transition start;
+       }
+    };
+    """
+  let program = try #UseOkResult(Program.Compile(simple_parser_declaration))
+  let cg = try #UseOkResult(CodeGenerator().codeGen(program))
+
+  let expected = shrink(
+    """
+    [
+        {
+            name: "main_parser",
+            init_state: "start",
+            parse_states: [
+                {
+                    name: "start",
+                    transitions: []
+                },
+            ]
+        }
+    ]
+    """)
+
+  #expect(expected == cg.getGeneratedCode())
+}
+
+@Test func test_codegen_parser_state_select_transition() async throws {
+  let simple_parser_declaration = """
+    parser main_parser() {
+       state start {
+            transition select (false) {
+              true: reject;
+              false: reject;
+           };
+       }
+    };
+    """
+  let program = try #UseOkResult(Program.Compile(simple_parser_declaration))
+  let cg = try #UseOkResult(CodeGenerator().codeGen(program))
+
+  /// TODO: Fix this test.
+  let expected = shrink(
+    """
+    [
+        {
+            name: "main_parser",
+            init_state: "start",
+            parse_states: [
+                {
+                    name: "start",
+                    transitions: [Select]
+                },
+            ]
+        }
+    ]
+    """)
+
+  #expect(expected == cg.getGeneratedCode())
+}