Fix recursive deepJoin and flattenDeepArray traversal hangs

[He-Pin]

Motivation

std.deepJoin and std.flattenDeepArray can hang on recursive arrays because the old traversal recursed without tracking the active array path. They should terminate with a normal builtin error instead of looping indefinitely.

Modification

• Add a shared iterative deep-array traversal helper with explicit stack frames.
• Track the active array path by identity so recursive arrays are reported deterministically.
• Keep fast handling for flat arrays and direct backing-array access for materialized arrays.
• Add regression tests for recursive and shared-array traversal cases.

Result

Case	go-jsonnet v0.22.0	jrsonnet 0.5.0-pre99	sjsonnet before	sjsonnet after
std.deepJoin(local a = [a]; a)	max stack frames exceeded	stack overflow / recursion failure	hangs / exceeds timeout	[std.deepJoin] max stack frames exceeded
std.flattenDeepArray(local a = [a]; a)	max stack frames exceeded	stack overflow / recursion failure	hangs / exceeds timeout	[std.flattenDeepArray] max stack frames exceeded

Flat and acyclic nested arrays still produce the same values as before.

Risks

• The traversal code is shared by two builtins, so regressions could affect both std.deepJoin and std.flattenDeepArray.
• Recursive arrays now fail earlier with a deterministic error instead of consuming time until the process is interrupted.