feat: emit a nicer panic when thread count overflows MAX_SHARDS (#64)

* Emit a nicer panic when thread count overflows `MAX_SHARDS`

Tackles #62 (comment)

* Replace `if cond { panic` with `assert!`

TODO: negate the condition in the assertion

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

* Negate the condition in the assertion

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

src/shard.rs

@@ -292,6 +292,13 @@ where
         let tid = Tid::<C>::current();
         test_println!("current: {:?}", tid);
         let idx = tid.as_usize();
+        assert!(
+            idx < self.shards.len(),
+            "Thread count overflowed the configured max count. \
+            Thread index = {}, max threads = {}.",
+            idx,
+            C::MAX_SHARDS,
+        );
         // It's okay for this to be relaxed. The value is only ever stored by
         // the thread that corresponds to the index, and we are that thread.
         let shard = self.shards[idx].load(Relaxed).unwrap_or_else(|| {