rt(unstable): add spawn Location to TaskMeta

[hawkw]

Motivation

As described in issue #7411, task spawning APIs are currently annotated
with #[track_caller], allowing us to capture the location in the user
source code where the task was spawned. This is used for tracing
events used by tokio-console and friends. However, this information is
not exposed to the runtime on_task_spawn, on_before_task_poll,
on_after_task_poll, and on_task_terminate hooks, which is a shame,
as it would be useful there as well.

Solution

This branch adds the task's spawn location to the TaskMeta struct
provided to the runtime's task hooks. This is implemented by storing a
&'static Location<'static> in the task's Core alongside the
task::Id. In [this comment][1], @ADD-SP suggested storing the
Location in the task's Trailer.

I opted to store it in the Core instead, as the Trailer is intended
to store "cold" data that is only accessed when the task completes,
and not on every poll. Since the task meta is passed to the
on_before_task_poll and on_after_task_poll hooks, we would be
accessing the Trailer on polls if we stored the Location there.
Therefore, I put it in the Core, instead, which contains data that we
access every time the task is polled.

Closes #7411

[ADD-SP]

Overall looks good to me, I just left two comments, which can be addressed in follow-up PRs.

[hawkw]

As a follow-up, it occurs to me that it might also be useful to add tasks' spawn locations to the trace collected by the task dump code, but I'd prefer that be done in a separate branch.