associability of single_view (and associability preservation of some algorithms)

if input of these algorithms models AssociativeSequence, then output will model AssociativeSequence, because:
```
erase and erase_key use: iterator_range, joint_view
pop_back and pop_front use: iterator_range
```
```
iterator_range and joint_view preserve associability
```

if input of these algorithms models AssociativeSequence, then output will ***not*** model AssociativeSequence, because:
```
push_back and push_front use: single_view, joint_view
insert and insert_range use: iterator_range, single_view, joint_view
```
```
single_view does not model AssociativeSequence
```

### can fusion let single_view model AssociativeSequence?