func (p Prependable) View() View {
	return p.buf[p.usedIdx:]
}
View returns a View of the backing buffer that contains all prepended
data so far.
package buffer
type Prependable struct {
	buf View
	usedIdx int
}
func NewPrependable(size int) Prependable {
	return Prependable{buf: NewView(size), usedIdx: size}
}
func NewPrependableFromView(v View) Prependable {
	return Prependable{buf: v, usedIdx: 0}
}