(main~>lib.h)+header for data structures,~migrate lib.h

+some file handlers for writing and reading (may help with very large
renders by doing them piecemeal when I can)

main.c

@@ -13,19 +13,7 @@
 #include <raylib.h>
 #include <raymath.h>
 
-struct State
-{
-  // Sandpiles
-  unsigned char *data;
-  size_t dwidth;
-
-  size_t window_len;
-  int multiplier;
-
-  bool thread_alive;
-};
-
-typedef struct State state_t;
+#include "./lib.h"
 
 void step(state_t *state)
 {