cursor.go
func (c *Cursor) FormatMask(mask rune) string {
	r := make([]rune, len(c.input))
	for i := range r {
		r[i] = mask
	}
	return format(r, c)
}
func (c *Cursor) FormatMask(mask rune) string {
	r := make([]rune, len(c.input))
	for i := range r {
		r[i] = mask
	}
	return format(r, c)
}
// FormatMask replaces all input runes with the mask rune.