Transform deeply nested param containers into a flat array of [key,
value] pairs.

*Examples:*

  >> flatten_params({key1: {key2: 123}})
  => [["key1[key2]", 123]]

  >> flatten_params({key1: {key2: 123, arr: [1,2,3]}})
  => [["key1[key2]", 123], ["key1[arr][]", 1], ["key1[arr][]", 2], ["key1[arr][]", 3]]

[Parameters:]

  - object (Hash, ParamsArray) -- The container to flatten
  - uri_escape (Boolean) -- Whether to URI escape keys and values
  - parent_key (String) -- Should not be passed (used for recursion)