The `Entered` guard returned by `Span::enter` represents entering and
exiting a span _on the current thread_. Calling `Span::enter` enters the
span, returning an `Entered`, and dropping the `Entered` ensures that
the span is exited. This ensures that all spans, once entered, are
eventually exited.

However, `Entered` has an auto-impl of `Send`, because it doesn't
contain any `!Send` types. This means that the `Entered` guard _could_
be sent to another thread and dropped. This would cause the original
thread to never exit the span,. and the thread to which the `Entered`
guard was sent would exit a span that it never observed an enter for.
This is incorrect.

This PR adds a `*mut ()` field to `Entered` so that it no longer
implements `Send`. There's now a manual `Sync` impl so structs holding
an `Entered` can still be `Sync`.

Fixes #698

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

thanks @MikailBag for the suggestion!

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

…o eliza/0.2-api-fixes

This reverts commit 81e9e5b. .

Ut turns out this approach doesn't actually work in this specific case.
Because `tracing` has conditional support for `no_std`, we can't use the
`std::sync` version of `MutexGuard`. I thought we could use the
`MutexGuard` from the spinlock implementation that's used as a fallback
on no-std platforms, but I'd forgotten that this is defined in
`tracing-core`, not `tracing`, and isn't publicly visible. I'll probably
go back to the previous approach of just accepting the additional
`unsafe` keyword necessary to manually impl `Sync`.