Full support for CHICKEN eggs in Nix

This pull request implements full support for CHICKEN Scheme in Nix. 

For this, we had to patch CHICKEN to support ```CHICKEN_REPOSITORY_EXTRA``` - a search path for multiple chicken repositories. The patch itself isn't upstream yet, but will hopefully superseded in the next release.

```fetchegg``` uses ```chicken-install``` to retrieve eggs. ```eggDerivation``` is used to build any CHICKEN egg installable via ```chicken-install```. A setup-hook is used to make the eggs available.

We also wrote the [egg2nix](https://github.com/the-kenny/egg2nix) tool which can generate nix-expressions for a given list of chicken eggs including all their dependencies, just like npm2nix does for node packages.

I'm submitting this as a Pull Request to get some feedback on the implementation of the various parts - comments are welcome!