Rename equality options to equalityFn #540

The comparison option has three names: compare in usePrevious, equalityFn in useControlledState, and areValuesEqual in usePreservedReference. All three have the same type, (a, b) => boolean.

• Rename the parameter to equalityFn in usePrevious and usePreservedReference. Both are positional, so only types and docs change.
• Change the default in usePrevious from === to Object.is, which useControlledState and React already use. Only NaN and -0 behave differently.
• Keep the deep comparison default in usePreservedReference, because keeping the reference of deeply equal values is what the hook is for.

Update types, docs, and tests.