Filesystem stuff on Windows

Minor image acquire/present refactor
Unsigned integer limits
Fixed typo in arena push/push copy macro

code/os/core.h

@@ -1,6 +1,11 @@
 #if !defined(LD_OS_CORE_H_)
 #define LD_OS_CORE_H_
 
+typedef struct OS_Handle OS_Handle;
+struct OS_Handle {
+	U64 v[1];
+};
+
 // Virtual memory
 
 function U64 VM_PageSize();
@@ -11,4 +16,6 @@ function B32   VM_Commit(void *base, U64 size);
 function void  VM_Decommit(void *base, U64 size);
 function void  VM_Release(void *base, U64 size);
 
+#include "filesystem.h"
+
 #endif  // LD_OS_CORE_H_