fix numpy scalar hash for bigendian (#285)

* Revert "KeyBuilder: use system byteorder to match numpy"

This reverts commit cee50279.

* fix numpy hash for bigendian

* speed up endianess determination

* cleanups, add mock test

* broaden test slightly

Co-authored-by: Michał Górny <mgorny@gentoo.org>

* rename _IS_BIGENDIAN

* add comments

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Co-authored-by: Michał Górny <mgorny@gentoo.org>