Make dependency on "failure" optional

[prokopyl]

While being compatible with failure is nice, the way it is currently done in this library forces the dependency on all users of this crate, even if they do not use it at all.

The biggest example of this issue is bindgen, the biggest dependent of this crate but which does not use failure, hence making everybody that uses bindgen depend on failure (and all its dependencies) for pretty much nothing.

This PR makes the dependency on failure optional, by creating a new use_failure feature that controls whether or not the dependency on failure is enabled.
This feature is enabled by default to avoid any breaking changes, but it can be disabled by any dependent crate that does not wish to use failure and can get rid of the extra dependency.

The CI configurations were also updated, so they run tests in both configurations: with and without default features enabled.

[harryfei]

Thanks for this PR.

[prokopyl]

You're welcome!

Do you plan to release a new version on crates.io soon? I would like to upstream those changes to bindgen (and maybe a few others) as soon as possible. 🙂

[harryfei]

@prokopyl I will release a new version today.

[prokopyl]

@harryfei Great, thank you!

[harryfei]

@prokopyl New version (3.0.0) has been published.