go/ast: change embedded token.Position fields to named fields

This is in preparation for a different position representation.
It also resolves situations where a node would be printed as
it's node position simply because the embedded token.Position
has a String method.

R=r
CC=golang-dev
https://golang.org/cl/2991041

src/cmd/cgo/ast.go

@@ -174,7 +174,7 @@ func (f *File) saveExport(x interface{}, context string) {
 
 		name := strings.TrimSpace(string(c.Text[9:]))
 		if name == "" {
-			error(c.Position, "export missing name")
+			error(c.Pos(), "export missing name")
 		}
 
 		f.ExpFunc = append(f.ExpFunc, &ExpFunc{

src/cmd/godoc/snippet.go

@@ -80,7 +80,7 @@ func genSnippet(d *ast.GenDecl, id *ast.Ident) *Snippet {
 	}
 
 	// only use the spec containing the id for the snippet
-	dd := &ast.GenDecl{d.Doc, d.Position, d.Tok, d.Lparen, []ast.Spec{s}, d.Rparen}
+	dd := &ast.GenDecl{d.Doc, d.Pos(), d.Tok, d.Lparen, []ast.Spec{s}, d.Rparen}
 
 	return newSnippet(dd, id)
 }

src/pkg/go/typechecker/typechecker.go

@@ -51,7 +51,7 @@ func CheckPackage(pkg *ast.Package, importer Importer) os.Error {
 //
 func CheckFile(file *ast.File, importer Importer) os.Error {
 	// create a single-file dummy package
-	pkg := &ast.Package{file.Name.Name, nil, map[string]*ast.File{file.Name.Position.Filename: file}}
+	pkg := &ast.Package{file.Name.Name, nil, map[string]*ast.File{file.Name.NamePos.Filename: file}}
 	return CheckPackage(pkg, importer)
 }