Add error types and simple state

src/main.c

@@ -1,7 +1,10 @@
 #include <stdio.h>
+#include "nihilispm.h"
+#include "nihilispm_state.h"
 
 int main(int argc, const char **argv) {
     (void) argc, (void) argv;
+    struct nl_state *state = nl_state_create();
 
     return 0;
 }