Flow type helpers

还没有人认领这个 Issue。

Flow offer some nice type helpers in his recent versions, while some of them are already addressed by typescript, some could perhaps be helpful.
Here are extracted comments from flow source just for inspiration.
- $Either<...T> is the union of types ...T
- $All<...T> is the intersection of types ...T
- $Supertype acts as any over supertypes of T
- $Subtype acts as any over subtypes of T
- $Shape matches the shape of T
- $Diff
- $Enum is the set of keys of T
- $Record is the type of objects whose keys are those of T

未指定源文件、测试或入口点。首先比较列出的 Flow helper 与 TypeScript 现有的类型系统功能以及 issue 讨论；完成条件是确定一致的 helper 范围、明确行为，并完成相应的实现和测试。

由索引模型根据 Issue 内容生成。