Add CFFI binding for the public API

Following up on https://github.com/search?q=repo%3Agoogle%2Fxls+Remove+Python+extensions&type=commits that removed the pybind11 extensions, it would be nice to go back to the approach described in https://github.com/google/xls/issues/108#issue-682499715 and enrich the public API currently at https://github.com/google/xls/tree/main/xls/public with CFFI bindings.