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.y

@@ -749,6 +749,25 @@ FORMAT {
   else
     $$ = BLOCK(gen_subexp(gen_const(jv_object())), $2, gen_op_simple(POP));
 } |
+/*
+ * This `$$$$varname` hack is strictly private to jq builtins.  DO NOT USE!!
+ *
+ * This is used in `_modify`, in src/builtin.jq, to avoid holding on to a
+ * reference to `.`.
+ *
+ * We could just have the compiler emit bytecode for `_modify` so it can use
+ * LOADVN w/o needing jq syntax for LOADVN.
+ *
+ * This syntax, `$$$$varname`, violates referential transparency: it has
+ * side-effects that are surprising.
+ *
+ * DO NOT USE!!  I will break your jq code if you do use this outside
+ * src/builtin.jq.
+ */
+'$' '$' '$' '$' IDENT {
+  $$ = gen_location(@$, locations, gen_op_unbound(LOADVN, jv_string_value($5)));
+  jv_free($5);
+} |
 '$' IDENT {
   $$ = gen_location(@$, locations, gen_op_unbound(LOADV, jv_string_value($2)));
   jv_free($2);