Configure required features for examples

[robinkrahl]

Instead of manually checking for required features in the hyphenation
and termwidth examples, we can configure the required features in
Cargo.toml. This makes the examples easier to read and maintain.

[mgeisler]

Oh yes, that would be much better!

[robinkrahl]

Hmm, it seems to break the CI. I’ll try to fix it.

[robinkrahl]

I’ve added some imports that I had accidentally removed, now it’s working again.

[mgeisler]

Thanks, that's much nicer!

[mgeisler]

Hi Robin,

Just a FYI in case you run into this too... :-) I got a weird error when trying to publish the crate after this:

% cargo publish --dry-run
    Updating crates.io index
   Packaging textwrap v0.15.0 (/home/mg/src/textwrap)
   Verifying textwrap v0.15.0 (/home/mg/src/textwrap)
error: failed to verify package tarball

Caused by:
  failed to parse manifest at `/home/mg/src/textwrap/target/package/textwrap-0.15.0/Cargo.toml`

Caused by:
  can't find `hyphenation` example at `examples/hyphenation.rs` or
  `examples/hyphenation/main.rs`. Please specify example.path if you
  want to use a non-default path.

I could fix it by specifying the path explicitly: #444.