include: Readd circular dependency between tree.h and parser.h There are dozens of downstream projects that only include tree.h but use declarations from parser.h. This broke after the recent cleanup of circular dependencies. Make tree.h include parser.h again. This is a hack but doesn't change the include directory struture. This commit only made it into the 2.12 branch but wasn't applied to master, so the issue turned up in 2.13.0 again. Should fix #734.