Avoid mentioning source types of object literals in error messages

Whenever a type that originates from an object literal is encountered, we should avoid writing out the whole type. It's rarely useful, and can get painful in a lot of contexts (think React Native styles objects).

- Type '{ goobledeyGock: string, tickTock: number, wakkaFlocka: { stuff: string } }' is not assignable to 'OneType | AnotherType'.
+ This object is not assignable to 'OneType | AnotherType'.

TypeScript typechecker から始め、調査対象の動作として issue の object-literal diagnostic example を使用します。object-literal source types がどのように識別されるか、また diagnostic でそれらの完全な構造をどのように省略すべきかを特定します。短い表現に一致しつつ、有用な target-type 情報を保持できれば完了です。