go/scanner: return literal as string instead of []byte

Removed many string conversions in dependent code.
Runs all tests. No change to gofmt output.

R=r
CC=golang-dev
https://golang.org/cl/4291070

src/cmd/cgo/gcc.go

@@ -1156,7 +1156,7 @@ func (c *typeConv) Opaque(n int64) ast.Expr {
 func (c *typeConv) intExpr(n int64) ast.Expr {
 	return &ast.BasicLit{
 		Kind:  token.INT,
-		Value: []byte(strconv.Itoa64(n)),
+		Value: strconv.Itoa64(n),
 	}
 }