First draft of Eigen::Tensor support (#4201)

* First draft of Eigen::Tensor support

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* Fix build errors

* Weird allocator stuff?

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* Remove unused + additional allocator junk

* Disable warning

* Use constexpr

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* clang tidy fixes

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* Resolve comments

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* Remove auto constexpr function

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* Try again for older C++

* Oops forgot constexpr

* Move...