go/types: support for customizable Alignof, Sizeof

(Offsetof is a function of Alignof and Sizeof.)

- removed IntSize, PtrSize from Context (set Sizeof instead)
- GcImporter needs a Context now (it needs to have
  access to Sizeof/Alignof)
- removed exported Size field from Basic (use Sizeof)
- added Offset to Field
- added Alignment, Size to Struct

R=adonovan
CC=golang-dev
https://codereview.appspot.com/7357046