Add font infrastructure

Sources/SwiftUIMath/Internal/Helpers/KeyBox.swift

@@ -0,0 +1,22 @@
+import Foundation
+
+final class KeyBox<Value: Hashable>: NSObject {
+  let wrappedValue: Value
+
+  init(_ wrappedValue: Value) {
+    self.wrappedValue = wrappedValue
+  }
+
+  override var hash: Int {
+    var hasher = Hasher()
+    hasher.combine(wrappedValue)
+    return hasher.finalize()
+  }
+
+  override func isEqual(_ object: Any?) -> Bool {
+    guard let other = object as? KeyBox<Value> else {
+      return false
+    }
+    return wrappedValue == other.wrappedValue
+  }
+}