Consider fallback for processors without SSE 4.1

[silasdavis]

I completely understand that this may be a non-goal for this project, but it would be useful for our usage if go-hex would fallback to stdlib encoding/hex when the processor does not support the necessary instructions.

When I adopted go-hex I didn't take the time to appreciate that this codec will fail on older processors. I might not have made the leap if I realised it would exclude some processors/people from running my code. It looks like processors dating to before 2007/8 would be affected by this. For reference here is our bug report that alerted us to this: hyperledger-archives/burrow#905

I guess this depends on how hard it would be to introduce a fallback and whether that would incur significant additional runtime cost, which I am not sure about. Please feel free if to just close this issue if you consider it out of scope - I just thought it was worth raising.

[tmthrgd]

A decade old CPU isn't something I exactly expected to come across. The fix isn't too difficult so I will include it.