Optimised Poly1305 loading code

Optimised Poly1305 loading code

By actually *rolling* the loading code.  I haven't looked at the
assembly, but I suspect the loop is easier for the compiler to
vectorise.

This results in a 5% speed increase on my machine (Intel i5 Skylake
laptop, gcc 7.3.0).

This fix was made possible by @Sadoon-AlBader on GitHub, who submitted
pull request #118