Add clone to Data.Array.ST

[gbagan]

Description of the change

This PR adds clone :: forall h a. STArray h a -> ST h (STArray h a) to the Data.Array.ST module.
It makes a mutable copy of a mutable array.
This is similar to the clone function in the Haskell module Data.Vector.Mutable.
This is equivalent to unsafeThaw <<< freeze but more readable.
I need it in one of my projects.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[garyb]

Could you add a note to the changelog too?

[garyb]

Thanks!

[garyb]

Now released as v7.3.0, apologies for the delay 🙂