Upgrading array view to span

@trivialfis has a PR here https://github.com/dmlc/xgboost/pull/3548 implementing the span class from the cpp core guidelines and the c++20 draft. Ideally we would replace the dmlc-core array_view class with this but dmlc-core currently has no cuda continuous integration testing, which is critical to our implementation. This issue is a placeholder for possibly implementing this in dmlc-core in the future.

Start by reviewing the dmlc-core array_view implementation and the linked XGBoost PR #3548 for its span approach. Confirm the required CUDA continuous-integration coverage before defining the replacement scope; done means a compatible span implementation is validated by the relevant CUDA tests.

Written by the indexing model from the issue text.