Improve TypedArray#map allocation (#821)

Author: MaxGraey

std/assembly/typedarray.ts

@@ -1464,13 +1464,19 @@ function MAP<TArray extends ArrayBufferView, T>(
   array: TArray,
   fn: (value: T, index: i32, self: TArray) => T,
 ): TArray {
-  var length = array.length;
+  var len = array.length;
   var dataStart = array.dataStart;
-  var out = instantiate<TArray>(length);
-  var outDataStart = out.dataStart;
-  for (let i = 0; i < length; i++) {
+
+  var byteLength = len << alignof<T>();
+  var out = changetype<TArray>(__alloc(offsetof<TArray>(), idof<TArray>()));
+  var buffer = __alloc(byteLength, idof<ArrayBuffer>());
+  out.data = changetype<ArrayBuffer>(buffer); // retain
+  out.dataStart = buffer;
+  out.dataLength = byteLength;
+
+  for (let i = 0; i < len; i++) {
     store<T>(
-      outDataStart + (<usize>i << alignof<T>()),
+      buffer + (<usize>i << alignof<T>()),
       fn(load<T>(dataStart + (<usize>i << alignof<T>())), i, array)
     );
   }