Add Core::FIFOCache

[SimBe195]

Cache based on an unordered_map with a maximum size such that when the cache is full and
a new item is trying to be added, the oldest item in the cache gets removed.
Note: oldest is determined by order of insertion, not order of last access.

Example:

FIFOCache<int, std::string> cache(2);  // Cache has room for two items
cache.put(1, "one");  // Internal data: {1: "one"}
cache.put(2, "two");  // Internal data: {1: "one", 2: "two"}
cache.put(3, "three");  // Oldest element is deleted since max size was reached. Internal data: {2: "two", 3: "three"}

[curufinwe]

This throws an exception if the key is not present. As nobody ever catches these it will probably terminate the application. I think it would be better to return std::optional<Value&> and for an interface the default-constructs the value add an operator[] function.

For the version returning std::optional<Value&> we could also have a const version of the function.

[SimBe195]

C++ unfortunately doesn't support optional references so std::optional<Value&> doesn't work . I have implemented this now by using std::optional<std::reference_wrapper<Value>>. It could also be done by using Value* as return value instead and returning nullptr when the key is not found.

[curufinwe]

PS: same comment about adding inline as in #78