new features:
- pkg.NewConstDecl: support iota
- unsafe.Sizeof
- unsafe.Alignof
- unsafe.Offsetof
- unsafe.Add
- unsafe.Slice
- gop parser: support mode=ParseGoFiles
- gop run support -gop: parse a .go file as a .gop file
- gop run: check isDirty to speed up
- gop run -nr: don't run if no change
- goptestgo: run all cases in GOROOT
changes: