$ObjMap property modifiers

[goodmind]

Related: #7639, #7571

Summary

Adds third argument to $ObjMap

Example

type Id = <T>(T) => T;

type Partial<T: {}> = $ObjMap<T, Id, { optional: true }>;
type Required<T: {}> = $ObjMap<T, Id, { optional: false }>;

type Readonly<T: {}> = $ObjMap<T, Id, { polarity: '+' }>;
type Writeonly<T: {}> = $ObjMap<T, Id, { polarity: '-' }>;
type ReadWrite<T: {}> = $ObjMap<T, Id, { polarity: 'N' }>;

// Regular $ObjMap still works

type Foo = $ObjMap<{|a: string|}, () => number>

[goodmind]

@jbrown215 would you review this after you're done with spreads project instead of $Required pull request? Because #7571 was closed.
If you still work on spreads project then nevermind.

[goodmind]

@jbrown215 anything about this?

[gkz]

We are working on implementing a version of mapped types instead of going with the approach of adding more features to $ObjMap.