Several classes have a lot of non-trivial members within them, or don't
but likely should have the destructor defaulted in the cpp file for
future-proofing/being more friendly to forward declarations.
Leaving the destructor unspecified allows the compiler to inline the
destruction code all over the place, which is generally undesirable from
a code bloat perspective.
std::shared_ptr isn't strictly necessary here and is only ever used in
contexts where the object doesn't depend on being shared. This also
makes the interface more flexible, as it's possible to create a
std::shared_ptr from a std::unique_ptr (std::shared_ptr has a
constructor that accepts a std::unique_ptr), but not the other way
around.
