Add jq builtin reduce_keys(f)

reduce_keys

reduce_keys(.foo) takes as input an array of objects, and produces a single object with values added for a same key 

 Example

        jq 'reduce_keys(.)'
Input	"[{"test-a": ["a"]}, {"test-a": ["b"]}, {"test-a": ["c"]}, {"test-b": ["x"]}, {"test-b": ["y"]}]"
Output	"{"test-a":["a","b","c"], "test-b":["x","y"]}"
---
        jq 'reduce_keys(.)'
Input	"[{"test-a": 1}, {"test-a": 2}, {"test-a": 3}, {"test-b": 4}, {"test-b": 5}]"
Output	"{"test-a":6, "test-b":9}"
---
        jq 'reduce_keys(.)'
Input	"[{"test-a": "a"}, {"test-a": "b"}, {"test-a": "c"}, {"test-b": "x"}, {"test-b": "y"}]"
Output	"{"test-a":"abc", "test-b":"xy"}"
---

builtin.c

@@ -914,6 +914,7 @@ static const char* const jq_builtins[] = {
  "def min(f): _min_by_impl(map([f]));",
  "def max_by(f): max(f);",
  "def min_by(f): min(f);",
+ "def reduce_keys(f): reduce .[] as $item ({}; .[($item|keys[])] += $item[] );",
 #include "libm.h"
  "def add: reduce .[] as $x (null; . + $x);",
  "def del(f): delpaths([path(f)]);",