chicken: Implement support for bootstrap-builds

This is necessary in order to apply custom patches, as chicken needs a
bootstrap build to regenerate *.c files. This is also necessary when
building from git.
Infrastructure to build chicken eggs.
Implement patches.nix to override parts of a derivation
rename patches -> overrides
Add chicken to PATH of wrapped chicken binaries.
New package: egg2nix.

Generates nix-expressions from Chicken Scheme Eggs.
eggDerivation: Move postInstall into installPhase.
Pass chickenEggs as a parameter to egg2nix.

Those eggs are specific to egg2nix - it's unwise to make them public.
Wrap chicken so it can find gcc.
egg2nix: Update to 0.2.

Also regenerate chicken-eggs.nix - the changeset is quite big, but
subsequent changes will be smaller as egg2nix now sorts its output.
Update egg2nix to 0.3.
Merge remote-tracking branch 'refs/remotes/origin/master' into chicken-eggs
egg2nix: Update to 0.4.

Let's hope this is the latest version.
egg2nix: Fix transitive dependencies.

These are also dependencies of http-client, but egg2nix uses them itself
too.
Merge pull request #4682 from the-kenny/chicken-eggs

Full support for CHICKEN eggs in Nix