wip: working on extern

- Feat: stabilize extern header
- Feat: add externs to walker
- Feat: add extern node walking logic

include/compiler/fzl_walker.h

@@ -1,6 +1,7 @@
 #ifndef FZL_WALKER_H
 #define FZL_WALKER_H
 
+#include <common/fzl_extern.h>
 #include <common/fzl_opcodes.h>
 #include <common/fzl_types.h>
 #include <compiler/fzl_parser.h>
@@ -73,6 +74,15 @@ typedef struct
 
   fzl_ctable_t *classes[64];
   size_t classes_count;
+
+  fzl_extern_lib_t extern_libs[64];
+  size_t extern_libs_count;
+
+  fzl_extern_type_t extern_types[256];
+  size_t extern_types_count;
+
+  fzl_extern_fn_t extern_fns[256];
+  size_t extern_fns_count;
 } fzl_walker_t;
 
 // Allocate new walker