Fold operation (code/docs/test)

testdata

@@ -331,6 +331,10 @@ def fac: if . == 1 then 1 else . * (. - 1 | fac) end; [.[] | fac]
 # []
 # 1001
 
+fold 0 as $s (.[] | $s + .)
+[1,2,4]
+7
+
 #
 # Assignment
 #