Use humantime crate in main

[Luthaf]

We currently manually format the elapsed time manually in the output log: https://github.com/lumol-org/lumol/blob/0f6efbd38b0bf132af2f912c0b233e5a8829bf02/src/bin/lumol.rs#L66-L87

This code could be replaced by using the humantime crate instead.