Fast implementation of data structures, including a key-value store, stack,
and queue. Environments are commonly used as key-value stores in R, but
every time a new key is used, it is added to R's global symbol table,
causing a small amount of memory leakage. This can be problematic in cases
where many different keys are used. Fastmap avoids this memory leak issue
by implementing the map using data structures in C++.
