wip: working on walker (externs)

Begin working on extern walking (libs metadata collet + opcode gen)

include/common/fzl_opcodes.h

@@ -20,8 +20,9 @@ typedef enum
   FZL_OPCODE_JMP = 0xF20001, // Jump to specified IP address
   FZL_OPCODE_JZ  = 0xF20002, // Jump if top value in dstack is zero
 
-  FZL_OPCODE_CALL = 0xF30001, // Call function
-  FZL_OPCODE_RET  = 0xF30002, // Return from function
+  FZL_OPCODE_CALL        = 0xF30001, // Call function
+  FZL_OPCODE_RET         = 0xF30002, // Return from function
+  FZL_OPCODE_CALL_EXTERN = 0xF30003, // Call external function
 
   FZL_OPCODE_ALLOC_OBJECT  = 0xFA0001, // Allocate N slots
   FZL_OPCODE_GET_FIELD     = 0xFA0002, // Get field N of an object at dstack

include/compiler/utils/fzl_extern.h

@@ -0,0 +1,16 @@
+#ifndef FZL_EXTERN_H
+#define FZL_EXTERN_H
+
+// Library symbol descriptor
+typedef struct
+{
+  //
+} fzl_extern_item_t;
+
+// External library descriptor
+typedef struct
+{
+  //
+} fzl_extern_lib_t;
+
+#endif // FZL_EXTERN_H