- Replaced raw pointers with references or smart pointers. Nullable
references are represented by `stmp::optional<const T&>`. (There are
many raw pointers still remaining. They should be replaced at some
point.)
- Added class template specializations `stmp::optional<T &>` and
`stmp::optional<const T&>`.
- Fixed `stmp::optional`'s various behaviors
- `World::{players, playerPersistents}` are now `std::array`.
- More uses of `stmp::optional` to clarify the semantics
- Renamed `PlayerThrownGrenade` to `PlayerThrewGrenade`
- Replaced old-style `for` loops with range based ones
- Deleted `Player`'s default constructors and `operator =`
- Deleted `TCGameMode`'s default constructor and `operator =`
- Deleted `CTFGameMode`'s default constructor and `operator =`
- Replaced `static_cast` with `dynamic_cast` for down-casting
- `RefCountedObject::operator*()` no longer requires non-constness to
return `T &`.
- Replaced the uses of `std::vector::operator[]` with `std::vector::at`
for bounds checking.
- Made some methods of `GameMap` `const`.
- Added some null checks.
