Introduce Rust code into Biotite

Resolves #688.

This PR introduces Rust code into Biotite. To make it work along the existing Cython modules the build backend is switched back to setuptools, using setuptools-rust to build the Rust modules.

The Rust code ends up in a single compiled file, accessible via biotite.rust. However this module contains multiple submodules mimicing the structure of biotite itself. For example there is biotite.rust.structure.io.pdb.

As initial module this PR integrates fastpdb into Biotite, including some refactoring.