Multiple functions can cause double-frees

The following functions in the crate are affected:

IdMap::clone_from

The clone_from implementation for IdMap drops the values present in the
map and then begins cloning values from the other map. If a .clone() call
pancics, then the afformentioned dropped elements can be freed again.

get_or_insert

get_or_insert reserves space for a value, before calling the user provided
insertion function f. If the function f panics then uninitialized or
previously freed memory can be dropped.

remove_set

When removing a set of elements, ptr::drop_in_place is called on each of the
element to be removed. If the Drop impl of one of these elements panics then
the previously dropped elements can be dropped again.