Implement `std::string_view` in C++14

## Brief feature description

Backport a non-owing `std::string_view` from C++17 to C++14. It can be useful for avoid to pass a `char*` where you can easily run into out-of-bounds problems, which is avoided using `string_view`:

```cpp
void foobar(char* myString)
{
// blablabla
}
```

## Detailed information

* [ ] Define API & possibly design document
* [ ] Implementation