(see issue #1235)

Fixes #1235. As seen in that issue, a syntax node can actually cross the
border of a None-delimited group, due to such groups being transparent
to the parser in most cases. In the cases that this can occur, the
presence of the group is known to be semantically irrelevant, so we just
explicitly ignore the presence of the group.

It's important that we only ignore None-delimited groups when the
verbatim end requires entering the group, as such groups can have semantic
impact (such as grouping binary operator chains) in other positions.

Implement `PartialOrd` for `Cursor`

    error: casting `usize` to `isize` may wrap around the value
      --> src/buffer.rs:56:46
       |
    56 |                     entries.push(Entry::End(-(group_end_index as isize)));
       |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_wrap
       = note: `-D clippy::cast-possible-wrap` implied by `-D clippy::pedantic`

    error: casting `usize` to `isize` may wrap around the value
      --> src/buffer.rs:82:34
       |
    82 |         entries.push(Entry::End(-(entries.len() as isize)));
       |                                  ^^^^^^^^^^^^^^^^^^^^^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_wrap

Restrict PartialOrd comparison to only tokens in the same buffer