lp8.remove_object: Improve resilience.

Prevent error when unit has no [modifications].
lp8.remove_object: Add `fail_silently` parameter.

If this is truthy, no error will be raised if the unit does not have the
object.
Add modifications/{get_object{s,_tags,_cfgs}.
Add utils/{keys,values,ivalues}.
Add modifications/{object{s,_tags,_cfgs}}.
Add modifications/remove_objects.
Properly implement [remove_object].