[ggj][nestedsig] feat: add a TriFunction (#624)

* fix: fix dep ordering in Bazel dedupe rules

* feat: add a basic trie

* fix: add hashCode() and equals() to Field

* fix: add equals() and hashCode() for MethodArgument

* feat: add a TriFunction

Author: miraleung

src/main/java/com/google/api/generator/util/TriFunction.java

@@ -0,0 +1,28 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.api.generator.util;
+
+import java.util.Objects;
+import java.util.function.Function;
+
+@FunctionalInterface
+public interface TriFunction<A, B, C, R> {
+  R apply(A a, B b, C c);
+
+  default <V> TriFunction<A, B, C, V> andThen(Function<? super R, ? extends V> after) {
+    Objects.requireNonNull(after);
+    return (A a, B b, C c) -> after.apply(apply(a, b, c));
+  }
+}