* Remove `unwrap()` from code
Remove `unwrap()` from the code (except tests).
Replaced it by either using `expect()` or handling the error more
gracefully.
* Improve documentation
Add examples on how to use the library
Add explanaitions on the dual use of crate::datatypes::Term
Relates to #33
* Update README
* Add contributing guidelines
* directly hooked into the internal BDD-primitive functions
* has worse performance than the approach without the primitive functions
* foreseeable for easy problems
* surprise for more complex problems, because recursion can be
escaped much earlier
* ADD the default-feature "variablelist" to have a compile-flag to either
enable or disable this feature.
* updated rust to the current version
Update clap to version 3.1.3, so
- structopt gets obsolete
- Errors are differently named, therefore tests needed to be adapted
- updated Cargo.lock for the binary
Fixed an issue with clippy warnings
