Restructure project, add gebs_exists() and gebs_mkdir()

example/self_rebuild.c

@@ -0,0 +1,11 @@
+#include <stdio.h>
+#define GEBS_IMPLEMENTATION
+#include "../gebs.h"
+
+int main(int argc, char ** argv)
+{
+    gebs_rebuild_self(argc, argv, "cc", "-o", "self_rebuild", __FILE__);
+
+    printf("Hello world\n");
+    return 0;
+}