TCO to the max!

Close #446.

    Currently tested by disassembling and tracing various recursive jq
    programs by hand under valgrind.  An improved test framework that
    can test for errors and specific bytecode patterns is in
    development.

bytecode.h

@@ -83,6 +83,7 @@ struct bytecode {
 void dump_disassembly(int, struct bytecode* code);
 void dump_operation(struct bytecode* bc, uint16_t* op);
 
+int bytecode_operation_length(uint16_t* codeptr);
 void bytecode_free(struct bytecode* bc);
 
 #endif