Rewrite logic in `vlen` to drop NumPy build dependency

[jakirkham]

As noted in issue ( https://github.com/zarr-developers/numcodecs ), vlen had problems with read-only data

Internally this occurred because we assigned the input array to object[:], which expects mutability. Ideally we would just replace this with const object[:]. However Cython does not support this ( https://github.com/cython/cython/issues/2485 ). Over time various blockers have been resolved on that front, but it is not yet fixed

Fortunately an astute SciPy developer noted that const <fused_type>[:] is allowed ( https://github.com/scipy/scipy/pull/18192#pullrequestreview-1359581611 ). We use that same trick here

This in turn should allow us to continue to accept read-only arrays. Though this does so without a NumPy build dependency, which can introduce more complexity to the build process

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TODO:

• [ ] Unit tests and/or doctests in docstrings
• [ ] Tests pass locally
• [ ] Docstrings and API docs for any new/modified user-facing classes and functions
• [ ] Changes documented in docs/release.rst
• [ ] Docs build locally
• [ ] GitHub Actions CI passes
• [ ] Test coverage to 100% (Codecov passes)