Clean up the scheme code

Introduce an "entry" name to define alist with package/generation
parameters and values.  Rename everything, update the commentary and
docstrings accordingly.
Generalize info/list buffers

Make it possible to receive and display any type of information.
Indent ‘guix-define-buffer-type’ macro properly
More generalizations for list buffers

Add ‘guix-list-define-entry-type’ macro.
Add support for displaying generations
Minor fixes in functions for installed entries
Add support for displaying generation packages
Add ‘guix-info-time’ face
Add ‘guix-info-insert-action-button’ function
Add action buttons in a generation info buffer
Update README for generations