Add kj::arr() for specifying arrays easily. Requries C++17.

std::initializer_list is problematic because it insists that its elements be const, meaning among other things that you can't move from them. So, an std::initializer_list<kj::String> is often useless. With kj::arr you can do like:

    kj::Array<String> = kj::arr(kj::mv(string1), kj::mv(string2));

This requires C++17 due to the fold expression. This could maybe be worked around using some ugly recursion but I'm writing C++17 these days so meh.

c++/src/kj/array-test.c++

@@ -378,5 +378,12 @@ TEST(Array, ReleaseAsBytesOrChars) {
   }
 }
 
+#if __cplusplus > 201402L
+KJ_TEST("kj::arr()") {
+  kj::Array<kj::String> array = kj::arr(kj::str("foo"), kj::str(123));
+  KJ_EXPECT(array == kj::ArrayPtr<const kj::StringPtr>({"foo", "123"}));
+}
+#endif
+
 }  // namespace
 }  // namespace kj

c++/src/kj/array.h

@@ -822,6 +822,15 @@ inline Array<T> heapArray(std::initializer_list<T> init) {
   return heapArray<T>(init.begin(), init.end());
 }
 
+#if __cplusplus > 201402L
+template <typename T, typename... Params>
+inline Array<Decay<T>> arr(T&& param1, Params&&... params) {
+  ArrayBuilder<Decay<T>> builder = heapArrayBuilder<Decay<T>>(sizeof...(params) + 1);
+  (builder.add(kj::fwd<T>(param1)), ... , builder.add(kj::fwd<Params>(params)));
+  return builder.finish();
+}
+#endif
+
 }  // namespace kj
 
 #endif  // KJ_ARRAY_H_