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 optionaltypePartial<T>={[PinkeyofT]?: T[P];};// Make all properties in T readonlytypeReadonly<T>={readonly[PinkeyofT]: T[P];};// From T pick a set of properties KtypePick<T,KextendskeyofT>={[PinK]: T[P];}// Construct a type with a set of properties K of type TtypeRecord<Kextendsstring|number,T>={[PinK]: T;}

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

[ahejlsberg]

@mhegazy Comments?