[Proposal] Add Kernel.get_in/1

[Marc-André Lafortune]

There is `pop_in/2` and its macro form `/2`
There is `put_in/3` and its macro form `/2`
There is `update_in/3` and its macro form `/2`
There is `get_and_update_in/3` and its macro form `/2`
There is `get_in/2` but no macro form `/1`. Could we please add `get_in/1`?

[Marc-André Lafortune]

I just remembered that `nil[:x]` works, so it's probably just nicer to write `data[:foo][:bar]`; let's forget about this 😅