Optimize compilation time a bit, if json_fwd.hpp is available

examples/crypto_bot.cpp

examples/simple_bot.cpp

include/mtx/common.hpp

include/mtx/errors.hpp

include/mtx/events.hpp

include/mtx/events/aliases.hpp

include/mtx/events/avatar.hpp

include/mtx/events/canonical_alias.hpp

include/mtx/events/common.hpp

include/mtx/events/create.hpp

include/mtx/events/encrypted.hpp

include/mtx/events/encryption.hpp

include/mtx/events/event_type.hpp

include/mtx/events/guest_access.hpp

include/mtx/events/history_visibility.hpp

include/mtx/events/join_rules.hpp

include/mtx/events/member.hpp

include/mtx/events/messages/audio.hpp

include/mtx/events/messages/emote.hpp

include/mtx/events/messages/file.hpp