feat(tvix/derivation): serialize Nix Derivation

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

tvix/Cargo.toml

@@ -19,6 +19,7 @@
 
 members = [
   "cli",
+  "derivation",
   "eval",
   "eval/builtin-macros",
   "nar",