[go] Use left shift for power of 2, idiomatic Go

Per @sjakobi's [helpful nitpick][hn]

[hn]: http://exercism.io/submissions/8d3f228b46ce09990e4f68ea
[go] Update grains totalling with simple MaxUint64

Per @sjakobi's [insightful nitpick][inp].

[inp]: http://exercism.io/submissions/8d3f228b46ce09990e4f68ea

Submission: http://exercism.io/submissions/c3d4e9e5f3bf4522266c768a