Bugfix multimap size

[NavyaZaveri]

With the current implementation, this test fails:

  #[test]
    fn test_size() {
        let mut mmap = MultiMap::<i32, String>::new();
        mmap.insert(1, "abc".into());
        mmap.insert(1, "abc".into());
        mmap.delete(1, "abc".into());
        assert_eq!( mmap.size(),0)
    }

The reason for this is that counter was being incremented for every insert, even if the key-value pair was a duplicate.

Note: this PR is built on top of the previous PR #5