Add overloads for 'map' on tuple types

[DanielRosenwasser]

This PR fixes #6574.

Sometimes users have the expectation that calling map on a homogeneous tuple will create a tuple type of the same size, but with the element types appropriately mapped on. For instance:

interface Person {
    name: string;
}

declare let couple: [Person, Person];

// Should have type '[string, string]', but currently has type 'string[]'.
let names = couple.map(p => p.name);

Now that we have this-types, we can actually enable overloads that can only apply if the target object (the thing being dotted on) has a specific type.

I'd like to hear feedback from the team (especially @sandersn & @ahejlsberg) about whether this is a good idea. This fulfills a lot of users' expectations, but they could instead just augment Array<T> with these overloads if they're interested.

Also tagging @philpee2 who asked me about this yesterday.

[mhegazy]

👍 after fixing failing tests

[sandersn]

This is specialised for [T, T] tuples, which aren't that common. In #6574 @aluanhaddad makes two good points:

1. T[] isn't that much worse than [T, T] as the result of map unless
2. [T, T] is a commonly used structure, in which case a tuple is not that good of a representation for it.

Can't individuals augment the Array interface themselves if they want this type for map?