Remove unused redundant item constructor.
Extract itype lookup to item constructor to streamline code and cleanup unused item_constroller methods.
Clean up some includes and remove unused item_factory methods.
Add the new header to git.
Rename a VERY poorly named variable.
Stop passing nextinv into item constructor.
Eradicate game::nextinv and associated methods.
Use item_factory instead of itype in item constructors, and push artifact creation down into item_factory.
Generate a fake itype if we encounter an unknown id instead of failing.
Remove unnecessary manual checks for item type existence from iuse.cpp.
Remove a few more redundant itype checks.
Merge remote-tracking branch 'kevingranade/item-constructor-cleanup' into FrankenMod

Conflicts:
	src/construction.cpp
	src/item.cpp
	src/item.h
	src/item_factory.cpp
	src/item_factory.h
	src/iuse.cpp
	src/mapgen.cpp
	src/pickup.cpp