Micro optimizations

[shuding]

Make the code cleaner and more consistent:

• use the useState({})[1] pattern for re-rendering the component
• make the internal property names shorter in Cache
• make the key prefixes in useSWRInfinite 3 chars (ctx@, inf@, len@), and internal cache property name shorter
• make addRevalidator return the ubsubscriber

Closes #896.

[codesandbox-ci]

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Sandbox	Source
SWR-Basic	Configuration
SWR-States	Configuration
SWR-Infinite	Configuration

[huozhi]

generally lgtm. the lint warning is a fake one. disabling it or rollback to array destruction are both acceptable.
eslint hooks ignore the none array pattern for useState, haven't found any useful case to work around yet

[shuding]

@huozhi It seems like the linter didn't yell, so I think it's fine :)

[ranisalt]

@shuding care to point out what is the useState({})[1] trick? I'm curious on what it changes.

[shuding]

@ranisalt it's an implementation detail inside SWR, previously we are using const [,rerender] = useState(false) to create a re-render function, now it uses const rerender = useState({})[1].