Initial jq recurse() implementation

BFS for jq parity and imperative to avoid call stack limit.

stdlib-candidate/std-rfc/mod.nu

@@ -4,3 +4,4 @@ export module str.nu
 # commands
 export use fs.nu *
 export use set-env.nu *
+export use recurse.nu *

stdlib-candidate/std-rfc/recurse.nu

@@ -0,0 +1,23 @@
+export def main [
+    --filter: closure
+]: any -> list<any> {
+    mut todo = [$in]
+    mut done = []
+    while ($todo | length) > 0 {
+        # pop
+        let value = $todo | last
+        $todo = ($todo | drop)
+        # save
+        if $filter == null or ($value | do $filter $value) {
+            $done ++= [$value]
+        }
+        # push
+        $todo ++= (match ($value | describe --detailed | get type) {
+            table => ($value | values | flatten)
+            record => ($value | values)
+            list => $value
+            _ => continue
+        } | reverse)
+    }
+    $done
+}