Changed std::arch to core::arch use

[fabiosky]

This commit address the fact that there are some intrinsic functions that are in core::arch but not in std::arch. One example of this is _bswap: https://doc.rust-lang.org/core/arch/x86/fn._bswap.html

Converting the imports to core allows to not mix std::arch and core::arch import across modules, for a cleaner design when thinking about extensibility. It also opens the future possibility to have a no_std implementation of the library.

[nacho]

Looks good