Improvements

[shuding]

• fix the case that the args array is empty:
  useSWR([]) shouldn't fetch
• fix hashing primitive types:
  useSWR([null]) ≠ useSWR(["null"])
• add a Prefetching Data section to the readme
• fix data is always undefined first, even when cached #144 and remove useHydration, cause we don't have to worry about mismatching for now (currently there's no persistent cache like local storage or indexedDB. So in both SSR, Suspense and normal prerender + CSR, the hydration result will always match the rendered template).