nix-ninja: Incremental Builds with Nix

nix-ninja is an incremental build tool leveraging the Nix build system. It parses ninja.build files, generating a derivation per compilation unit, and uses content-addressed derivations for granular, Nix-native incrementality. It's compatible with the ninja CLI, supporting both local and Nix derivation execution. Currently experimental and reliant on unreleased Nix features, it already builds simple C++ examples and even Nix itself. Contributors are welcome; milestones 0.1.0 (correctness) and 0.2.0 (performance) are outlined.