Remove the insane "fold" operation, replace with saner "reduce".

builtin.c

@@ -554,10 +554,10 @@ static const char* const jq_builtins[] = {
   "def unique: group_by(.) | map(.[0]);",
   "def max_by(f): _max_by_impl(map([f]));",
   "def min_by(f): _min_by_impl(map([f]));",
-  "def add: fold null as $sum (.[] | $sum + .);",
+  "def add: reduce .[] as $x (null; . + $x);",
   "def del(f): delpaths([path(f)]);",
-  "def _assign(paths; value): value as $v | fold . as $obj (path(paths) as $p | $obj | setpath($p; $v));",
-  "def _modify(paths; update): fold . as $obj (path(paths) as $p | $obj | setpath($p; getpath($p) | update));",
+  "def _assign(paths; value): value as $v | reduce path(paths) as $p (.; setpath($p; $v));",
+  "def _modify(paths; update): reduce path(paths) as $p (.; setpath($p; getpath($p) | update));",
   "def recurse(f): ., (f | select(. != null) | recurse(f));",
   "def to_entries: [keys[] as $k | {key: $k, value: .[$k]}];",
   "def from_entries: map({(.key): .value}) | add;",