Go19 backward compatibility

[willf]

Add support for Go 1.9 bits, but keep backward compatibility

[coveralls]

Coverage remained the same at 96.893% when pulling e2d3b02 on go19-backward-compatibility into 988f4f2 on master.

[coveralls]

Coverage decreased (-0.006%) to 96.887% when pulling e50f633 on go19-backward-compatibility into 988f4f2 on master.

[coveralls]

Coverage decreased (-0.006%) to 96.887% when pulling 137ee1a on go19-backward-compatibility into 988f4f2 on master.

[coveralls]

Coverage decreased (-0.006%) to 96.887% when pulling b77e40d on go19-backward-compatibility into 988f4f2 on master.

[willf]

Hi @lemire

Here is a version that continues to support 1.8 (and maybe earlier, but the make qa script requires 1.8 or higher).

Which version do you think it would be better to keep, this or the just 1.9 version?

I think this is better, tho a bit sloppy. When the next version of Go comes out, I might change some function signatures and require 1.9.

[lemire]

I think that backward compatibility is good to have, at least temporarily. It is not like anyone has a strong incentive to stick with an old version of Go for years...