Clean up <include>s (#19)

* fixup includes

There are some unnecessary headers in `finders_interface.h` that emit
warnings. This commit aims to:
- remove unnecessary headers,
- move necessary headers to where they're needed, and include them
transitively, and
- "decorate" technically unused, yet necessary-for-usage headers
with `// IWYU pragma: export`.

* remove newline

* compile fix

Author: lyorig

example/main.cpp

@@ -1,5 +1,5 @@
 #include <iostream>
-#include <list>
+#include <utility>
 #include <rectpack2D/finders_interface.h>
 
 /* For description of the algorithm, please see the README.md */

src/rectpack2D/best_bin_finder.h

@@ -1,7 +1,7 @@
 #pragma once
 #include <variant>
 #include <cassert>
-#include <algorithm>
+#include <optional>
 #include "rect_structs.h"
 
 namespace rectpack2D {

src/rectpack2D/empty_space_allocators.h

@@ -1,8 +1,6 @@
 #pragma once
 #include <array>
 #include <vector>
-#include <type_traits>
-
 #include "rect_structs.h"
 
 namespace rectpack2D {

src/rectpack2D/empty_spaces.h

@@ -1,4 +1,5 @@
 #pragma once
+#include <optional>
 #include "insert_and_split.h"
 
 namespace rectpack2D {

src/rectpack2D/finders_interface.h

@@ -1,16 +1,8 @@
 #pragma once
-#include <optional>
-#include <vector>
-#include <array>
-#include <variant>
 #include <memory>
-#include <algorithm>
-
-#include "insert_and_split.h"
 #include "empty_spaces.h"
-#include "empty_space_allocators.h"
-
 #include "best_bin_finder.h"
+#include "empty_space_allocators.h" // IWYU pragma: export
 
 namespace rectpack2D {
 	template <class empty_spaces_type>

src/rectpack2D/rect_structs.h

@@ -1,5 +1,5 @@
 #pragma once
-#include <utility>
+#include <algorithm>
 
 namespace rectpack2D {
 	using total_area_type = int;
@@ -87,4 +87,4 @@ namespace rectpack2D {
 	};
 
 	using space_rect = rect_xywh;
-}
+}