[Decompile] file-io, loader-h, texture-h, level-h, math-camera-h (#397)

* file-io and loader-h

* add test for level-h

* math camera-h

test/decompiler/reference/all_forward_declarations.gc

@@ -292,3 +292,23 @@
 ;; vector
 ;; only because trig isn't in the reference yet.
 (define-extern deg-lerp-clamp (function float float float float))
+
+;; file-io
+(defenum file-kind
+    :bitfield #f
+    (level-bt 0) ;; aka bsp-header.
+    (art-group 1)
+    (tpage 2)
+    (dir-tpage 3)
+    (level-vs 4)
+    (tx 5)
+    (vis 6)
+    )
+
+;; loader-h
+(declare-type art-group basic)
+
+;; level-h
+(declare-type entity-links structure)
+(declare-type level-group basic)
+(define-extern *level* level-group)