Render overlapping marks

This renders overlapping multiline marks on the same lines of source code. This means that the following example:

```
error[E0308]: match arms have incompatible types

   ┌── codespan/src/file.rs:1:9 ───
   │
 1 │ ╭         match line_index.compare(self.last_line_index()) {
 2 │ │             Ordering::Less => Ok(self.line_starts()[line_index.to_usize()]),
 3 │ │             Ordering::Equal => Ok(self.source_span().end()),
 4 │ │             Ordering::Greater => LineIndexOutOfBoundsError {
 5 │ │                 given: line_index,
 6 │ │                 max: self.last_line_index(),
 7 │ │             },
 8 │ │         }
   │ ╰─────────' `match` arms have incompatible types
   ·
 2 │               Ordering::Less => Ok(self.line_starts()[line_index.to_usize()]),
   │                                 --------------------------------------------- this is found to be of type `Result<ByteIndex, LineIndexOutOfBoundsError>`
 3 │               Ordering::Equal => Ok(self.source_span().end()),
   │                                  ---------------------------- this is found to be of type `Result<ByteIndex, LineIndexOutOfBoundsError>`
 4 │               Ordering::Greater => LineIndexOutOfBoundsError {
   │ ╭──────────────────────────────────^
 5 │ │                 given: line_index,
 6 │ │                 max: self.last_line_index(),
 7 │ │             },
   │ ╰─────────────^ expected enum `Result`, found struct `LineIndexOutOfBoundsError`
   │
   = expected type `Result<ByteIndex, LineIndexOutOfBoundsError>`
        found type `LineIndexOutOfBoundsError`
```

…is now rendered as:

```
error[E0308]: match arms have incompatible types

   ┌── codespan/src/file.rs:1:9 ───
   │
 1 │   ╭         match line_index.compare(self.last_line_index()) {
 2 │   │             Ordering::Less => Ok(self.line_starts()[line_index.to_usize()]),
   │   │                               --------------------------------------------- this is found to be of type `Result<ByteIndex, LineIndexOutOfBoundsError>`
 3 │   │             Ordering::Equal => Ok(self.source_span().end()),
   │   │                                ---------------------------- this is found to be of type `Result<ByteIndex, LineIndexOutOfBoundsError>`
 4 │   │             Ordering::Greater => LineIndexOutOfBoundsError {
   │ ╭─│──────────────────────────────────^
 5 │ │ │                 given: line_index,
 6 │ │ │                 max: self.last_line_index(),
 7 │ │ │             },
   │ ╰─│─────────────^ expected enum `Result`, found struct `LineIndexOutOfBoundsError`
 8 │   │         }
   │   ╰─────────' `match` arms have incompatible types
   │
   = expected type `Result<ByteIndex, LineIndexOutOfBoundsError>`
        found type `LineIndexOutOfBoundsError`
```

Closes #100