nix_expr: Allow to use Nix-style paths in dicts.

Nix allows to specify nested attributes without actually need to nest
them syntactically, like this:

{ a.b.c.d = 123; }

In Python we're using tuples, so the same example looks like this:

{ ('a', 'b', 'c', 'd'): 123, }

It's still not as nice as in Nix, but at least we can now avoid
specifying all these nested dictionaries.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
deployment: Use paths in pretty-printed dicts.

Although this doesn't shorten line length significantly, however it
_does_ make some of them more readable and we avoid constructs like
"}}}}" for closing deeply nested single item attribute sets.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>