func (r *Ranges) Add(lo, hi uint64, val interface{}) {
	r.rs = append(r.rs, rangeEnt{lo, hi, val})
	r.sorted = false
}
Add inserts val for range [lo, hi).

Add is undefined if [lo, hi) overlaps a range already in r.
package perfsession
import "sort"
type Ranges struct {
	rs     []rangeEnt
	sorted bool
}
type rangeEnt struct {
	lo, hi uint64
	val    interface{}
}