ThreadLocal reuses a slot without clearing its old values #68

hx::thread::ThreadLocal (used by the Haxe 5 std Tls) recycles a slot id when its Tls is collected, but the values stored in that slot on each thread are never cleared. A new Tls that gets the recycled id reads the old owner's value, and that value stays alive until it is overwritten. The legacy __hxcpp_tls_* ids and ThreadLocal ids also both start at 0 and index the same per-thread array.

Related: HaxeFoundation/haxe#12967 (eval frees Tls values when the Tls dies).