runtime: improve goroutine profiler

[dvyukov]

Some concrete suggestions:
1. Add goroutine state (waiting, runnable, syscall) as comment in debug more and/or as
top frame (so that all waiting goroutines converge in a single "Waiting" block
in svg graph.
2. Add "created by" line in debug mode.
3. Dedup stacks, there can be thousands of equal stacks.

[dvyukov]

Comment 1:

Another useful piece of information is how long a goroutine is blocked (similar to what
we output in crash reports). But this conflicts with stack deduplication.

[hyangah]

Comment 2:

which debug mode do you target? Do you target to add additional display debug mode?
* debug=1 doesn't show each goroutine state or created-by info, but collapses the equal
stacks into a group.
* debug=2 shows each goroutine's state and created-by info along with function arguments.

[dvyukov]

Comment 3:

You are right, I've missed debug=2 mode.
So the only potential addition is 1.
I wanted to check how it would look like, but it turned out that pprof does not support
goroutine profiles. That's something to fix.

[rsc]

Comment 4:

Labels changed: added release-none, removed release-go1.4.