Dependency embeddings

The addition in this release is support for dependencies as context. This makes it possible to train dependency embeddings as described by Levy & Goldberg, 2014. The dependency embedding model can be tuned in fine-grained detail (such as the depth of the relations).

• Add dependency relations.
• Refactoring training to make it easier to add different context types.
• Precompiled releases, including a MUSL target.
• Migration to Rust 2018.
• ff-train has been renamed to ff-train-skipgram.