`file` should be template

For the same reason that C++ makes all streams templated with a `charT` parameter, `file` should become
```c++
template<class charT, class traits = std::char_traits<charT>>
class basic_file : public std::basic_iostream<charT, traits>
```

`file` must be aliased `basic_file<char>` and exported from the library to maintain ABI compatibility