LDL factorisation routine. Factorize and solve quasi-definite linear
systems of the form `Ax=b`.
Use of the source code is governed by the Apache License, Version 2.0
([LICENSE](LICENSE) or https://www.apache.org/licenses/LICENSE-2.0).
The original [C](https://github.com/osqp/qdldl) source code from OSQP
was translated into [Rust](https://rustlang.org/) by Richard Lincoln.