Add missing include directives (bugfix)

When defining -DUSE_JSON, I found that, in addition to is_bool not being defined for nlohmann::json (see #66 (closed)), there were various missing #include directives for several items from the standard library that prevented successful compilation. It turns out these were being magically picked up from variant.hpp! This MR adds the necessary includes (#include <sstream>, #include <deque>, etc...).