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>

Sources/P4Compiler/Walker.swift

@@ -0,0 +1,54 @@
+// 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 SwiftTreeSitter
+import TreeSitterP4
+
+public struct Walker {
+  var currentChildIdx: Int
+  let childCount: Int
+  let node: Node
+
+  public init(node: Node) {
+    self.currentChildIdx = 0
+    self.childCount = node.childCount
+    self.node = node
+  }
+
+  public mutating func next() {
+    self.currentChildIdx += 1
+  }
+
+  public func getNext() -> Node? {
+    // If it is safe, then return the node!
+    if self.currentChildIdx < self.childCount {
+      return self.node.child(at: self.currentChildIdx)!
+    }
+    return .none
+  }
+
+  public func overUntil(n: Int, todo: (Node) -> Result<()>) -> Result<()> {
+    for currentChildIdx in currentChildIdx..<n {
+      let currentChild = node.child(at: currentChildIdx)!
+      if case Result.Error(let e) = todo(currentChild) {
+        return Result<()>.Error(e)
+      }
+    }
+    return Result.Ok(())
+  }
+}