compiler, testing: Build System for Compilation

The compilation code was written as a precursor for implementation
with macros. The updates in this commit make the switch.

There is still plenty to do:

1. Comment Walker.
2. Comment Macros.

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

Package.swift

@@ -75,7 +75,8 @@ let package = Package(
         ),
         .testTarget(
             name: "Tests",
-            dependencies: ["P4Compiler", "P4Runtime", "P4Lang", "Macros", "TreeSitterExtensions", "Common"]
+            dependencies: ["P4Compiler", "P4Runtime", "P4Lang", "Macros", "TreeSitterExtensions", "Common"],
+            swiftSettings: [.enableExperimentalFeature("CodeItemMacros")],
         ),
     ],
 )