… BackendCurrently in Scala.js, varargs call like `List(1, 2, 3)`, it is translated into the IR form `js.WrappedArray(JSArrayConstr(...))`. That requires JS interop for constructing the array and accessing its elements.Since Wasm-to-JS calls are expensive, this is undesirable for performance.This commit experiments avoiding `JSArrayConstr` for varargs.Instead, varargs are transformed into something like `new WrappedArray$ofInt(ArrayValue(1, 2, 3))` (or `new ArraySeq$ofInt(...)` on 2.13) to explore potential Wasm-specific optimizations.Note1: While reducing JS interop can improve performance on the Wasm backend, the same does not apply the JS backend.We'd need to re-optimize back to `JSArrayConstr`-based code during the Optimizer for the JS backend.Note 2: How about `WrappedArray.make` instead of directly instantiating specialized `WrappedArray`?I found that runtime type checks in `make` appear to be very slow, and in some micro-benchmarks, using `make` performed slightly worse than the original `JSArrayConstr`-based code.---Unfortunately, the performance improvements were negligible.For example, in the following code:```scaladef main(args: Array[String]): Unit = {  val startTime = System.nanoTime()  val xs = Seq(1, 2, ..., 20)  xs.foreach(x => assert(x > 0))  val endTime = System.nanoTime()  println(s"elapsed: ${endTime - startTime} ns")}```Both versions (with and without `JSArrayConstr`) reported similar timings of ~540000–580000 ns.Benchmarks run using [`sjrd/scalajs-benchmarks/wasm`](https://github.com/sjrd/scalajs-benchmarks/tree/wasm) also did not show any significant performance differences.| Benchmark   | before       | after      | Ratio (after / before) ||-------------|--------------|--------------|----------------------|| sha512      | 12403.95816  | 12737.42497  | 1.0269               || sha512int   | 12259.02363  | 13313.69655  | 1.0860               || queens      | 2.954778067  | 2.920396237  | 0.9873               || list        | 60.56316878  | 60.52163829  | 0.9993               || richards    | 87.49714448  | 87.77807725  | 1.0032               || cd          | 32866.35461  | 31672.2486   | 0.9637               || gcbench     | 104672.8678  | 121351.2553  | 1.1588               || tracerFloat | 870.5680015  | 876.4962162  | 1.0068               || tracer      | 784.3968099  | 783.2297365  | 0.9985               || sudoku      | 3634.165046  | 3609.813857  | 0.9933               || nbody       | 23722.03084  | 24192.39211  | 1.0198               || permute     | 262.9023071  | 265.949228   | 1.0116               || deltaBlue   | 525.4683864  | 511.8722724  | 0.9742               || kmeans      | 206339.5187  | 202615.0022  | 0.9820               || brainfuck   | 2352.518883  | 2357.064959  | 1.0019               || json        | 288.1723513  | 280.1001847  | 0.9720               || bounce      | 33.12443674  | 33.01821262  | 0.9978               |There may still be room for further optimization in the non-`JSArrayConstr` implementation.