Partial elimination of TMP from xutils.hpp and patches for dependencies

[spectre-ns]

Checklist

• The title and commit message(s) are descriptive.
• Small commits made to fix your PR have been squashed to avoid history pollution.
• Tests have been added for new features or bug fixes.
• API of new functions and classes are documented.

Description

With concepts and constexpr the library can be more readable and less reliant on recursion. This PR is the first of hopefully many that will flatten the template structure in xtensor logic without changing public interfaces or functionality. The goal is to make additional optimizations and functionality easier to reason about and implement moving forward.

Additionally, eliminating recursion should help compile times!

[codspeed-hq]

Merging this PR will not alter performance

✅ 255 untouched benchmarks

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_{Comparing spectre-ns:sfinae-to-constexpr (62d19f4) with master (1b5325b)}

[spectre-ns]

@Alex-PLACET @JohanMabille please let me know if these types of changes are in the direction you'd like to see xtensor move towards!