feat: implement AST walker

include/common/fzl_opcodes.h

@@ -0,0 +1,12 @@
+#ifndef FZL_OPCODES_H
+#define FZL_OPCODES_H
+
+// Available opcodes
+typedef enum
+{
+  FZL_OPCODE_LOADCONST,   // Load const into T (temp) reg
+  FZL_OPCODE_PUSHARG,     // Push argument from T into args stack
+  FZL_OPCODE_CALL_BUILTIN // Call builtin function
+} fzl_opcode_t;
+
+#endif // FZL_OPCODES_H