Fix accidentally quadratic behavior in _modify

We need to be careful to not grab an extra reference when mutating data
structures because that means we make extra copies.  Doing that every
time in `_modify` is really painful, as that function implements `|=`
and all modify-assign operators.

`jv_setpath()` also grabs additional references, so this is not the only
fix needed for the modify-assign operators to not be accidentally
quadratic.

We have to use `LOADVN` in order to make this fix possible, so we should
really byte-code `_modify` rather than jq-code it.  However, as a
stop-gap to make this fix easier, I'm adding syntax for referring to a
`$binding` such that you get `LOADVN` instead of `LOADV`.

This syntax is not meant to be used outside jq's internals!  Therefore
it is not documented.  I promise to break it later, so don't use it!

TBD, but later:

 - Optimize `_modify` for the case where `update` outputs more than one
   value.

   This is trivial: add a `setpath($p; null)` in the middle of `_modify`
   before calling `update`.  But unfortunately the VM retains a
   reference to `value_at_path` for path expression checking, and fixing
   that will require more work.

src/parser.h

@@ -1,9 +1,8 @@
-/* A Bison parser, made by GNU Bison 3.3.2.  */
+/* A Bison parser, made by GNU Bison 3.0.4.  */
 
 /* Bison interface for Yacc-like parsers in C
 
-   Copyright (C) 1984, 1989-1990, 2000-2015, 2018-2019 Free Software Foundation,
-   Inc.
+   Copyright (C) 1984, 1989-1990, 2000-2015 Free Software Foundation, Inc.
 
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -31,9 +30,6 @@
    This special exception was added by the Free Software Foundation in
    version 2.2 of Bison.  */
 
-/* Undocumented macros, especially those whose name start with YY_,
-   are private implementation details.  Do not rely on them.  */
-
 #ifndef YY_YY_SRC_PARSER_H_INCLUDED
 # define YY_YY_SRC_PARSER_H_INCLUDED
 /* Debug traces.  */
@@ -44,7 +40,7 @@
 extern int yydebug;
 #endif
 /* "%code requires" blocks.  */
-#line 11 "src/parser.y" /* yacc.c:1927  */
+#line 11 "src/parser.y" /* yacc.c:1909  */
 
 #include "locfile.h"
 struct lexer_param;
@@ -61,7 +57,7 @@ struct lexer_param;
     }                                           \
   } while (0)
 
-#line 65 "src/parser.h" /* yacc.c:1927  */
+#line 61 "src/parser.h" /* yacc.c:1909  */
 
 /* Token type.  */
 #ifndef YYTOKENTYPE
@@ -167,12 +163,12 @@ struct lexer_param;
 
 union YYSTYPE
 {
-#line 31 "src/parser.y" /* yacc.c:1927  */
+#line 31 "src/parser.y" /* yacc.c:1909  */
 
   jv literal;
   block blk;
 
-#line 176 "src/parser.h" /* yacc.c:1927  */
+#line 172 "src/parser.h" /* yacc.c:1909  */
 };
 
 typedef union YYSTYPE YYSTYPE;