runtime: capture per-m trace state in a type

runtime: capture per-m trace state in a type

More tightening up of the tracer's interface.

While we're here, clarify why waittraceskip isn't included by explaining
what the wait* fields in the M are really for.

Change-Id: I0e7b4cac79fb77a7a0b3ca6b6cc267668e3610bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/494190
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>