ds: add EraseIter to Set, MultiSet and MultiMap

[ynm3n]

Motivation

I wanted to delete a element one by one from MultiSet and MultiMap. However, Erase(K) delete all elements having key K.

Solution

Using EraseIter(iter), we can delete the element one by one.

Example

package main

import (
	"fmt"

	"github.com/liyue201/gostl/ds/set"
	"github.com/liyue201/gostl/utils/comparator"
)

func main() {
	ms := set.NewMultiSet(comparator.IntComparator)

	// Insert
	for i := 0; i < 3; i++ {
		ms.Insert(5000)
	}
	fmt.Println(ms) // [5000 5000 5000]

	// EraseIter (one element)
	it := ms.Find(5000)
	ms.EraseIter(it)
	fmt.Println(ms) // [5000 5000]

	// Erase (all elements)
	ms.Erase(5000)
	fmt.Println(ms) // []
}

Other little things

Map already have had EraseIter, so I did nothing to that.

I am ready to send a pull request related this issue. If you approve of this feature addition, I will send that to this repository.