fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.format())
    }
Given a `Span`, `fmt` will print out where the
`Span` occurs in its source. Single-line
`Span`s:
```ignore
12 | x = blatant { error }
|     ^^^^^^^^^^^^^^^^^
```
Multi-line `Span`s:
```ignore
12 > x -> {
13 >    y = x + 1
14 >    another { error }
15 > }
```
passerine-common/src/span.rs