Add support for tuples

[luis-j-soares]

This is likely too close to Generics support, but we could use some way to define arrays with fixed element count and types.

Typescript does it with tuple types:

type StringNumberPair = [string, number];

function doSomething(pair: StringNumberPair) {
  const a = pair[0];
//      ^ const a: string
  const b = pair[1];
//      ^ const b: number

  // ...
}
 
doSomething(["hello", 42]);

I think the syntax here could be the same as Typescript's.