Add length to Data.Array.ST

[Blugatroff]

Description of the change

This PR adds length :: forall h a. STArray h a -> ST h Int to the Data.Array.ST module.
This was recently requested in #238 and i need it as well.

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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)

[JordanMartinez]

@garyb Can I get a second approval here?

[garyb]

I'd suggest we just export a foreign length without the lengthImpl? The Impl stuff is only needed if we're doing something like maybe-ifying an FFI operation.

[garyb]

Thanks!