Project revamp

[infinisil]

This implements the plan in #61 plus some more. That is:

• Only unstable HIE
• Only GHC 8.6.5 and 8.8.3 (or more in the future), but on Linux for both glibc 2.27 (NixOS 19.09) and glibc 2.30 (NixOS 20.03/unstable)
• Using haskell.nix for the builds. This makes evaluation slower, but makes things simpler overall.
• No global installation intended anymore, instead you add HIE to your projects shell.nix file
• Fully working template projects are provided in the templates directory, including one using haskell.nix and one using the nixpkgs infrastructure
• All builds and cachix pushes are now done using GitHub Actions (previously I built and pushed everything manually). The templates are also tested like that.

There are still some small things to do:

• The templates are both cabal-based, maybe a stack-based one would be nice too
• The templates should get a small Readme explaining how/why it works and how to use it
• Attempting to use GHC versions other than 8.6.5 or 8.8.3 will probably fail, since haskell.nix doesn't fully support them. An early error might be better than an obscure haskell.nix error

[infinisil]

I have now done all the things I wanted and I think this PR is ready. CI takes a long time, but once it passes I'll merge this. If any problems are encountered with these changes, feel free to open issues for them.

With this merged, these issues can be closed:

• Provide an overlay #6 because an overlay is the only way all-hies is distributed now
• Switch to haskell.nix #19 because this uses haskell.nix now
• Error in emacs: can't load .so/.DLL for libm.so.6 due to GLIBC_2.29 #32 because on Linux the glibc version is now matched
• Does this tool allow to install hie with hoogle docs? #47 because the templates now have a working hoogle database setup
• Update to 1.2 #52 and New version(s) #57 because the version is now finally updated to 1.4
• Support GHC 8.8.3 #55 because 8.8.3 is now one of the supported versions
• bundle Cabal #58 because the templates demonstrate how a working Cabal version can be obtained
• Please, provide explicit description of the project status #60 and The future of all-hies #61 because the project is now updated

[Anton-Latukha]

Thank you for all this.