Predefined mapped types in lib.d.ts

[ahejlsberg]

This PR adds the following predefined mapped types to lib.d.ts:

// Make all properties in T optional
type Partial<T> = {
    [P in keyof T]?: T[P];
};

// Make all properties in T readonly
type Readonly<T> = {
    readonly [P in keyof T]: T[P];
};

// From T pick a set of properties K
type Pick<T, K extends keyof T> = {
    [P in K]: T[P];
}

// Construct a type with a set of properties K of type T
type Record<K extends string | number, T> = {
    [P in K]: T;
}

The PR also property implements the identity relation for mapped types.

[ahejlsberg]

@mhegazy Comments?