Elm: automate packaging with elm2nix

- rewrite documentation
- automate whole process into update.sh
- remove all legacy snippets

pkgs/development/compilers/elm/README.md

@@ -0,0 +1,24 @@
+# To update Elm:
+
+Modify revision in ./update.sh and run it
+
+# Notes about the build process:
+
+The elm binary embeds a piece of pre-compiled elm code, used by 'elm
+reactor'. This means that the build process for 'elm' effectively
+executes 'elm make'. that in turn expects to retrieve the elm
+dependencies of that code (elm/core, etc.) from
+package.elm-lang.org, as well as a cached bit of metadata
+(versions.dat).
+
+The makeDotElm function lets us retrieve these dependencies in the
+standard nix way. we have to copy them in (rather than symlink) and
+make them writable because the elm compiler writes other .dat files
+alongside the source code. versions.dat was produced during an
+impure build of this same code; the build complains that it can't
+update this cache, but continues past that warning.
+
+Finally, we set ELM_HOME to point to these pre-fetched artifacts so
+that the default of ~/.elm isn't used.
+
+More: https://blog.hercules-ci.com/elm/2019/01/03/elm2nix-0.1/