Drop byte_utils in favor of native `to/from_be_bytes` methods

Drop byte_utils in favor of native `to/from_be_bytes` methods

Now that our MSRV supports the native methods, we have no need
for the helpers anymore. Because LLVM was already matching our
byte_utils methods as byteswap functions, this should have no
impact on generated (optimzied) code.

This removes most of the byte_utils usage, though some remains to
keep the patch size reasonable.