trace: reduce the number of the printing APIs

[rakyll]

The trace package currently allows users to print in a few different ways:

func LazyPrint(ctx context.Context, str fmt.Stringer)
func LazyPrintWithAttributes(ctx context.Context, attributes []Attribute, str fmt.Stringer)
func LazyPrintf(ctx context.Context, format string, a ...interface{})
func LazyPrintfWithAttributes(ctx context.Context, attributes []Attribute, format string, a ...interface{})
func Print(ctx context.Context, str string)
func PrintWithAttributes(ctx context.Context, attributes []Attribute, str string)

Instead of giving users different combinations of these options, we should provide a printing entry type and allow user to provide their options via the new type.

type Entry struct {
	Text       fmt.Stringer
	Attributes []Attribute
}

func LazyPrint(ctx context.Context, e Entry)

Then, we can provide maybe one other option for convience:

func LazyPrintf(ctx context.Context, format string, a ...interface{})

Users who doesn't want lazy formatting can always implement their own fmt.Stringer to format it at the point LazyPrint is called.

/cc @jcd2 @bogdandrutu