Fix list struct print bug

[billylanchantin]

Part of: #1014

Fixes an issue where we can't print columns with a dtype like {:list, {:struct, ...}} where the root of the tree isn't a :struct but it contains a :struct. The cause is that we're not accounting for nested structs in our print function:

explorer/lib/explorer/data_frame.ex

Lines 5982 to 5990 in 07f80fb

	|> Enum.map(fn n ->
	s = df[n]
	list = Series.to_list(s)
	case s.dtype do
	{:struct, _} -> Enum.map(list, &inspect/1)
	_ -> list
	end
	end)

The solution is a recursive function.

I've also made some additional choices:

1. Contents of structs now look the same inside and outside a struct.
   Before we were calling inspect/1. That resulted in e.g. strings getting "" delimiters. Strings don't usually get "" delimiters when printed in a dataframe.
2. Lists now get [] delimiters.
   This is especially helpful in identifying empty lists and singleton lists. Before these cases were a bit confusing.
3. Structs now get {} delimiters instead of %{}.
4. Keys of structs go from "x" => to x:.
5. nil is now represented as the literal nil instead of an empty string.
6. We now intent one space when nesting.

Overall I found these choices looked much better. Here's just one example of the new format:

+--------------------------------------------------------+
|       Explorer DataFrame: [rows: 3, columns: 2]        |
+--------------------------------+-----------------------+
|             col_d              |         col_j         |
|          <struct[2]>           |   <list[list[f32]]>   |
+================================+=======================+
| {                              | [                     |
|  q: 2901-09-24 22:53:38.923913 |  [                    |
|  w: 3470490443432017527        |   1.100000023841858   |
| }                              |   2.200000047683716   |
|                                |  ]                    |
|                                | ]                     |
+--------------------------------+-----------------------+
| {                              | [[3.299999952316284]] |
|  q: nil                        |                       |
|  w: nil                        |                       |
| }                              |                       |
+--------------------------------+-----------------------+
| {                              | [                     |
|  q: nil                        |  nil                  |
|  w: 3594148901191965499        |  [nil]                |
| }                              |  []                   |
|                                | ]                     |
+--------------------------------+-----------------------+

[josevalim]

All is good to me!

Keys of structs go from "x" => to x:

The idea would be to show what it would actually be used if they were converted to Elixir, but either way is fine!

[billylanchantin]

I decided on x: over "x" => because 1) horizontal space is limited and 2) we don't use literals elsewhere. E.g. strings are my string not "my string"

If later anyone feels strongly about these choices though I'm happy to walk them back!