runtime: capture per-g trace state in a type

runtime: capture per-g trace state in a type

More tightening up of the tracer's interface.

This increases the size of each G very slightly, which isn't great, but
we stay within the same size class, so actually memory use will be
unchanged.

Change-Id: I7d1f5798edcf437c212beb1e1a2619eab833aafb
Reviewed-on: https://go-review.googlesource.com/c/go/+/494188
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>