This adds a Derivation structure and allows to write it to a structure that implements std::fmt:Write.
The implementation is based on the go-nix version.
Change-Id: Ib54e1202b5c67f5d206b21bc109a751e971064cf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7659
Reviewed-by: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
