util: add track_caller to public APIs (#4785)

* util: add track_caller to public APIs

Functions that may panic can be annotated with `#[track_caller]` so that
in the event of a panic, the function where the user called the
panicking function is shown instead of the file and line within Tokio
source.

This change adds `#[track_caller]` to all the non-unstable public APIs in
tokio-util where the documentation describes how the function may panic
due to incorrect context or inputs.

In one place, an assert was added where the described behavior appeared
not to be implemented. The documentation for `DelayQueue::reserve`
states that the function will panic if the new capacity exceeds the
maximum number of entries the queue can contain. However, the function
didn't panic until a higher number caused by an allocation failure. This
is inconsistent with `DelayQueue::insert_at` which will panic if the
number of entries were to go over MAX_ENTRIES.

Tests are included to cover each potentially panicking function.

Refs: #4413

* fix tests on FreeBSD 32-bit (I hope)

Some tests were failing on FreeBSD 32-bit because the "times too far in
the future" for DelayQueue were also too far in the future for the OS.

Fixed by copying the MAX_DURATION value from where it's defined and
using it to create a duration that is just 1 more than the maximum. This
will start to break once we get close (within 2 and a bit years) of the
Epochalypse (19 Jan, 2038) - but a lot of other things are going to be
breaking on FreeBSD 32-bit by then anyway.

tokio-util/src/task/spawn_pinned.rs

@@ -57,7 +57,9 @@ impl LocalPoolHandle {
     /// pool via [`LocalPoolHandle::spawn_pinned`].
     ///
     /// # Panics
+    ///
     /// Panics if the pool size is less than one.
+    #[track_caller]
     pub fn new(pool_size: usize) -> LocalPoolHandle {
         assert!(pool_size > 0);
 
@@ -167,6 +169,7 @@ impl LocalPoolHandle {
     /// }
     /// ```
     ///
+    #[track_caller]
     pub fn spawn_pinned_by_idx<F, Fut>(&self, create_task: F, idx: usize) -> JoinHandle<Fut::Output>
     where
         F: FnOnce() -> Fut,
@@ -196,6 +199,7 @@ struct LocalPool {
 
 impl LocalPool {
     /// Spawn a `?Send` future onto a worker
+    #[track_caller]
     fn spawn_pinned<F, Fut>(
         &self,
         create_task: F,
@@ -324,6 +328,7 @@ impl LocalPool {
         }
     }
 
+    #[track_caller]
     fn find_worker_by_idx(&self, idx: usize) -> (&LocalWorkerHandle, JobCountGuard) {
         let worker = &self.workers[idx];
         worker.task_count.fetch_add(1, Ordering::SeqCst);