Add a function to add a key/value pair to the dict only if it doesn't exist
within the dict already.
As the dict is type agnostic this requires a comparator function to be given.
Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>
As a dict can have multiple values for any given key we add a function to
query all occurences of a given key and return them as a (new) list.
Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>
