**BREAKING:**
- The `deepMerge` exported from payload now handles more complex data and
is slower. The old, simple deepMerge is now exported as `deepMergeSimple`
- `combineMerge` is no longer exported. You can use
`deepMergeWithCombinedArrays` instead
- The behavior of the exported `deepCopyObject` and `isPlainObject` may
be different and more reliable, as the underlying algorithm has changed
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