set::iterator should be constant

[`boost::container::set`'s `iterator`](https://github.com/boostorg/container/blame/develop/include/boost/container/set.hpp#L99) is a mutable iterator. This allows modification of keys, which can destroy the container's invariants, for example when using the erase-remove idiom.
This used to be a [defect](https://cplusplus.github.io/LWG/issue103) in `std::set` as well but has been fixed.
As can be seen in [cppreference](https://en.cppreference.com/w/cpp/container/set)

```
iterator | Constant LegacyBidirectionalIterator
const_iterator | Constant LegacyBidirectionalIterator
```

and in stdlib:
```C++
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// DR 103. set::iterator is required to be modifiable,
// but this allows modification of keys.
typedef typename _Rep_type::const_iterator iterator;
typedef typename _Rep_type::const_iterator const_iterator;
```