RoomStateView

This class is called to provide an arbitrary snapshot of a room state;
as the first step, Room::currentState() returns an instance of this
class that stores, well, the current state. Implelementation-wise it's
the same hash map of two-part state event keys to const event pointers;
however, RoomStateView provides additional operations:
- get(), that deprecates Room::getCurrentState(), returns a pointer to
  a particular event if the current state has it. Unlike the original
  method, the pointer returned from this one can be nullptr; this is
  done to get rid of stubbed state events that have to be created
  everytime a "state miss" occurred (i.e., when getCurrentState()
  does not find an existing event in the current state).
- eventsOfType() - this is a new place for Room::stateEventsOfType()
  introduced recently.
- query() - this is a way to specify a piece of the state content that
  you need to retrieve by passing a member function or a function object
  that retrieves it. That is especially convenient with member functions
  of the event class; just pass the pointer to this member function,
  and query() will parse the event type it has to retrieve out of it and
  call that member function on the event object. Returns an Omittable<>;
  if the respective piece of state doesn't exist, you'll get
  `Quotient::none` (the same as `std::nullopt`).
- queryOr() - the same but with the fallback value; instead of an
  Omittable<>, the fallback value will be returned if the needed event
  is not found.

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