refactor `_rewrite_return`
refactor `_rewrite_return`

* Cleanup the documentation
* Use a slice based implementation for block splitting

numba/numba,IntelLabs/numba,cpcloud/numba,numba/numba,seibert/numba,IntelLabs/numba,numba/numba,seibert/numba,seibert/numba,numba/numba,cpcloud/numba,IntelLabs/numba,numba/numba,seibert/numba,seibert/numba,IntelLabs/numba,IntelLabs/numba,cpcloud/numba,cpcloud/numba,cpcloud/numba