Missing NFData instances for unboxed arrays

[NFData](https://hackage.haskell.org/package/deepseq-1.5.0.0/docs/Control-DeepSeq.html#g:1) instances for unboxed arrays (`UArray`, `STUArray`, `IOUArray`, `StorableArray`) are currently missing.

Ideally, they would be defined here, but we can't do that because `deepseq` (which defines `NFData`) depends on `array`.
However, `deepseq` has agreed to drop this dependency (see https://github.com/haskell/deepseq/issues/102), which makes this possible in the future.

For `array`, this would mean:

1. Wait for a new `deepseq` release which does not depend on `array`
2. Depend on `deepseq` and define the instances.

Does this sound ok to the maintainers here?