[Ur] Nix-based Ur/Web building system

[Sergey Mironov]

Hi. Recently I repaired and document a build system for Ur/Web
applications called urweb-build. The goal of this project is to
develop safe, reliable and scalable method of building Ur/Web
applications and libraries.

https://github.com/grwlf/urweb-build

It is correct to think that urweb-build has properties of package
manager, but in fact all the packaging functionality is derived from
its dependencies: Git-Submodules and Nix (www.nixos.org/nix). The
expression itself consists mainly of 300-lines long URP-file generator
written in Nix expression language. With this expression installed,
one may replace *urp file with similar-looking build.nix file. As a
result, we get perfect NIx-powered library management, including the
ability to write custom build stages, using any preprocessors, etc.
And all this stuff will be (binary-)fetched on demand with only a
single command `nix-build`.

I already ported many libraries I wrote earlier and I am quite happy
with the tests, so I'd like to invite others to try it.

The demo project is https://github.com/grwlf/urweb-fviewer

Regards,
Sergey

PS
Uru3 and some other dependencies of Fviewer may require `cake3` - my
previous approach to Ur/Web building problem. Base urweb-build
expression contains rule for it, so Nix should handle the process
completely automatically. I don't plan to use cake3/Haskell in future
Ur/Web projects.