Integrate the logging code with `ErrorReport`

It would be pretty cool to be able to write something like

```c++
ErrorReport err = match_identifiers(...);
if (!err.ok()) {
   Log::fatal() << err;
}
```

or like

```c++
auto err = make_located_error("undefined variable", location, ErrorLevel::Fatal);
err.print();
```

and have it print `[ Fatal Error ] At xx:xx -- undefined variable`, followed by termination of the program.