Add fixed_shape::operator==

[tailsu]

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Description

Fixes #2376

[tdegeus]

Thanks!

To deal with the compiler issues I think you would just need to remove

xtensor/test/test_xreducer.cpp

Line 944 in a8f1a6a

bool operator==(fixed_shape<I...>, fixed_shape<J...>)

Furthermore, I think you still use std::equal internally

[tdegeus]

Actually I think that an implementation is already under way:

#2352