Improve single line label rendering

[brendanzab]

This makes the rendering of carets smarter when there are multiple labels on the same line. Instead of this:

   ┌─ one_line.rs:3:5
   │
 3 │     v.push(v.pop().unwrap());
   │     - first borrow later used by call
   │       ---- first mutable borrow occurs here
   │            ^ second mutable borrow occurs here
   │

...we now render the following:

   ┌─ one_line.rs:3:5
   │
 3 │     v.push(v.pop().unwrap());
   │     - ---- ^ second mutable borrow occurs here
   │     │ │
   │     │ first mutable borrow occurs here
   │     first borrow later used by call
   │

Resolves #211.

[brendanzab]

Ok, this should be ready for review!

[brendanzab]

This should ultimately address one of the main complaints described in #231 by @tnowacki. cc. diem/diem#3447

[brendanzab]

Note that this still does not cover the case where single line labels overlap multiline labels. Multiline labels will still be rendered below single line labels, eg.

40 │ │   Fruitcake jelly-o danish toffee. Tootsie roll pastry cheesecake
41 │ │   soufflé marzipan. Chocolate bar oat cake jujubes lollipop pastry
42 │ │   cupcake. Candy canes cupcake toffee gingerbread candy canes muffin
   │ │                  ------------- ^^^^^^^^^^^^^^^^^^ blah blah
   │ ╰──────────^ blah blah

I still think this is a much needed improvement, however!