style: Use DoubleEndedIterator::next_back

[caspermeijn]

Prevents clippy warnings like:

warning: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
   --> prost-build/src/code_generator.rs:473:54
    |
473 |                         .and_then(|ty| ty.split('.').last())
    |                                                      ^^^^^^ help: try: `next_back()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#double_ended_iterator_last
    = note: `#[warn(clippy::double_ended_iterator_last)]` on by default