pkg/units: Unit constants directly int64

int64 seems sufficient

Docker-DCO-1.1-Signed-off-by: Francisco Carriedo <fcarriedo@gmail.com> (github: fcarriedo)
pkg/units: Updated tests with unit constants

Also, now that I was at it, gave them a small refactor.

Docker-DCO-1.1-Signed-off-by: Francisco Carriedo <fcarriedo@gmail.com> (github: fcarriedo)