Free method
Frees the native memory owned by slot key and removes it from the
table.
Throws if key has not been allocated.
Implementation
void Free(String key) {
if (!_Slots.containsKey(key)) {
throw StateError('[FREE] Cannot free unallocated slot $key');
}
calloc.free(_Slots[key]!.$1);
_Slots.remove(key);
}