Replace mdspan with upstream libc++ implementation

[jrhemstad]

### Tasks
- [ ] Backport libc++ implementation to C++14 (no submdspan())
- [ ] Implement `submdspan_extents` and write exhaustive tests for it
- [ ] Implement the various `submdspan_mapping` specializations for the different mappings
- [ ] Implement `submdspan` and write exhaustive tests for it
- [ ] Implement `layout_left_padded` and write exhaustive tests for it
- [ ] Implement `layout_right_padded` and write exhaustive tests for it

[jrhemstad]

@miscco to fill in the description and breakdown of tasks.

[miscco]

Update06/2024:

• I have implemented and backported libc++ implementation and incorporated all tests into our test framework
• I have implemented submdspan_extents without proper testing

@bernhardmgruber is taking that over on a "when time permits" basis

[bernhardmgruber]

@miscco this should be largely done, right?

[miscco]

This is done, except for the padded layouts