TransientMap acts as a wrapper for std::collections::HashMap which allows for
the eviction of unused elements. In addition to the standard hashmap API, it provides
the following extra functions:
drain_unused removes all of the elements that have not been inserted or accessed
since the previous drain call, returning the elements as an iterator. The entirety of
the drain operation takes O(unused elements) time. Note that this is faster
than the amount of time required to iterate over the entire map, which is O(capacity).drain_used removes all of the elements that have been inserted or accessed since
the last drain call. The entirety of the drain operation takes O(used elements) time.set_all_used marks all elements as having been accessed in O(1) time.set_all_unused marks all elements as not having been accessed in O(1) time.These additional functions make TransientMap an ideal choice for applications like
caching, where it is desirable to efficiently discard data that has not been used.
The following is a brief example of how to use TransientMap:
```rust
let mut map = TransientMap::new();
map.insertunused(1, "a");
map.insertunused(2, "b");
asserteq!(Some("b"), map.remove(&2));
map.insert(3, "c");
map.insert(4, "d");
asserteq!(vec!((1, "a")), map.drain_unused().collect::
let mut res = map.drainunused().collect::
asserteq!(vec!((3, "c"), (4, "d")), res); asserteq!(0, map.len()); ```