BREAKING CHANGE: reactive arrays no longer unwraps contained refs
When reactive arrays contain refs, especially a mix of refs and
plain values, Array prototype methods will fail to function
properly - e.g. sort() or reverse() will overwrite the ref's value
instead of moving it (see #737).
Ensuring correct behavior for all possible Array methods while
retaining the ref unwrapping behavior is exceedinly complicated; In
addition, even if Vue handles the built-in methods internally, it
would still break when the user attempts to use a 3rd party utility
functioon (e.g. lodash) on a reactive array containing refs.
After this commit, similar to other collection types like Map and
Set, Arrays will no longer automatically unwrap contained refs.
The usage of mixed refs and plain values in Arrays should be rare in
practice. In cases where this is necessary, the user can create a
computed property that performs the unwrapping.
