Create "chainweb-portable" package, which can run on other linuxes

[ryantrinkle]

This has not been extensively tested, but it did work on the one Ubuntu machine I tried.

To build, do this: nix-build ./project.nix -A proj.ghc.chainweb-portable

To install, take all the files in the output and copy them to /opt/chainweb on the target machine. This prefix directory can be changed by supplying a different argument in project.nix, but it cannot be made fully relocatable easily. Perhaps something like rtldi could help, but I haven't evaluated it. It would also be possible to supply a script based on patchelf to change the prefix without rebuilding.

[ryantrinkle]

To clarify: I have tested that chainweb-node successfully starts, and provides output that looks like it's running, on Ubuntu 17.10 (Linux kernel 4.13.0). I haven't tested that the program works beyond that.

[ryantrinkle]

Converted to draft because it still depends on a fork of nixpkgs. PR to nixpkgs is up here: https://github.com/NixOS/nixpkgs/pull/88070