snappy: split into multiple outputs

Most of the size was documentation;
it got into many closures via mysql attribute.

pkgs/development/libraries/snappy/default.nix

···
     sha256 = "0mq0nz8gbi1sp3y6xcg0a6wbvnd6gc717f3vh2xrjmfj5w9gwjqk";
   };
 
+  outputs = [ "out" "dev" "doc" ];
+
   nativeBuildInputs = [ pkgconfig ];
 
   # -DNDEBUG for speed