Parser cleanup

r? @pcwalton

A month's worth of parser cleanup here. Much of this is new comments and renaming. A number of these commits also remove unneeded code.  Probably the biggest refactor here is splitting "parse_item_or_view_item" into two functions; it turns out that the only overlap between items in foreign modules and items in regular modules was macros, so this refactor should make things substantially easier for future maintenance.