WIP: fix Config.configure/6

[ibarakaiev]

Contributor checklist

• Bug fixes include regression tests
• Features include unit/acceptance tests

It seems that are few more bugs lurking in the configure/6 function (probably in the functions that it uses underneath). I can't quite get a hang on what's wrong and can't look into it further at the moment but just wanted to flag these.

Current behavior for the first test (config set to []) is:

import Config

config :foo,
       SomeModule,
       (
         []
         {:level1, [level2: :value]}
       )

and for the second is:

* (SyntaxError) invalid syntax found on nofile:3:43:
         error: syntax error before: '=>'
         │
       3 │ config :foo, SomeModule, level1: [:level2 => [level3: :value]]
         │                                           ^
         │
         └─ nofile:3:43