Allow Map to be initialized with an iterable

[pebrianz]

Feature suggestion

Right now, to create a map and fill it with data, we have to do it in two steps: first create an empty map, then add each item one by one with .set().

This is what our code currently looks like:

let map = new Map<string, i32>();
map.set('a', 1);
map.set('b', 2);

This works, but it can get a bit long and clunky, especially for a lot of data.

My suggestion is to allow the Map constructor to accept an array of key-value pairs. This would let us create and populate a map in a single, clean line of code.

Here's an example of what that would look like:

let map = new Map<string, i32>([
  ['a', 1],
  ['b', 2]
]);

This change would make our code much cleaner and easier to read, and it's a common pattern in other languages like JavaScript and typescript.

because assemblyscript not support mixed type like [K, V], we use entry object instead:

let map = new Map<string, i32>([
  {key: "a", value: 1},
  {key: "a", value: 2}
]}