Contrib/0/rfc169 proof/out

[KAction]

Supporting code for proposed RFC0169

nix-repl> cmus.override { alsaSupport = false; }
trace: warning: Feature parameter 'alsaSupport' is deprecated in favor of 'with_alsa'.
«derivation /nix/store/jm87wmcnpibrxfzwz95i51zh2qdxxp32-cmus-2.10.0.drv»

nix-repl> cmus.override { with_alsa = false; }
«derivation /nix/store/jm87wmcnpibrxfzwz95i51zh2qdxxp32-cmus-2.10.0.drv»

[KAction]

I did some compraison of evaluation time. As far as time concerned, I can't see any difference, but stats for nix-instantiate -A cmus say:

-  "nrFunctionCalls": 640435,
+  "nrFunctionCalls": 648806,

which means 1.3% more function calls. If I evaluate something big, but without anything renamed, like emacs, I get following:

-  "nrFunctionCalls": 451758,
+  "nrFunctionCalls": 457842,

which is also 1.3% more function calls.