fix a bug in thread spawning

src/primitive/defs.rs

@@ -2044,14 +2044,14 @@ primitive!(
  /// : wait≡spawn/+.
  ///
  /// For spawn threads in a thread pool, use [pool].
- ([1], Spawn, Thread, "spawn"),
+ ([1], Spawn, Thread, "spawn", Impure),
  /// Spawn a thread in a thread pool
  ///
  /// Has the same functionality as [spawn], but uses a thread pool instead of spawning a new thread.
  /// While [spawn]'s function will be called immediately, [pool]'s function will be called when a thread in the pool is available.
  /// The thread pool has as many threads as the machine has processors.
  /// If all threads in the pool are busy, then [pool] will block until a thread is available.
- ([1], Pool, Thread, "pool"),
+ ([1], Pool, Thread, "pool", Impure),
  /// Wait for a thread to finish and push its results to the stack
  ///
  /// The argument must be a thread id returned by [spawn] or [pool].
@@ -2065,13 +2065,13 @@ primitive!(
  /// [wait] is pervasive and will call [each] implicitly.
  /// ex: ↯3_3⇡9
  /// : wait≡spawn/+.
- (1, Wait, Thread, "wait"),
+ (1, Wait, Thread, "wait", Mutating),
  /// Send a value to a thread
  ///
  /// Expects a thread id returned by [spawn] or [pool] and a value to send.
  /// The thread id `0` corresponds to the parent thread.
  /// The sent-to thread can receive the value with [recv] or [tryrecv].
- (2(0), Send, Thread, "send"),
+ (2(0), Send, Thread, "send", Impure),
  /// Receive a value from a thread
  ///
  /// Expects a thread id returned by [spawn] or [pool].

src/run.rs

@@ -1438,7 +1438,7 @@ code:
  pub(crate) fn wait(&mut self, id: Value) -> UiuaResult {
  let ids = id.as_natural_array(self, "Thread id must be an array of natural numbers")?;
  if ids.shape.is_empty() {
- let handle = ids.data.into_iter().next().unwrap();
+ let handle = ids.data[0];
  #[cfg(not(target_arch = "wasm32"))]
  let thread_stack = self
  .rt