I.e. rewrite nix-build, download-from-binary-cache etc. in C++.

• scripts/nix-collect-garbage.in
• scripts/find-runtime-roots.pl.in
• scripts/nix-copy-closure.in
• scripts/nix-prefetch-url.in
• scripts/nix-install-package.in
• corepkgs/buildenv.pl
• scripts/nix-channel.in
• scripts/build-remote.pl.in
• scripts/nix-build.in
• scripts/nix-push.in
• scripts/download-from-binary-cache.pl.in

Darwin:

• scripts/resolve-system-dependencies.pl.in