go_spec: fixed a couple omissions/type errors
go_spec: fixed a couple omissions/type errors

- use math.Sqrt instead of Math.sqrt
- use float64 for Point fields to match math.Sqrt
- distinguish between Point and Point3D for clarity
- add alignment sizes for complex types

R=r, rsc, iant, ken2
CC=golang-dev
http://codereview.appspot.com/3420041
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go