New container: double ended string?

By analogy with `boost::container::devector`, I think it would be useful to have a `boost::container::destring`.

One of possible use cases is to create complicated concatenation chains without memory allocations(just my opinion, not sure does it worth, not sure does anybody really want it):

```
boost::container::destring str = "Hello world";
str.reserve(str.size() + 2); // prepare to insert one element front and yet one back
const auto braced_str = "(" + std::move(str) + ")"; // no memory allocation at all
```