runtime/trace: rename "Span" with "Region"

"Span" is a commonly used term in many distributed tracing systems
(Dapper, OpenCensus, OpenTracing, ...). They use it to refer to a
period of time, not necessarily tied into execution of underlying
processor, thread, or goroutine, unlike the "Span" of runtime/trace
package.

Since distributed tracing and go runtime execution tracing are
already similar enough to cause confusion, this CL attempts to avoid
using the same word if possible.

"Region" is being used in a certain tracing system to refer to a code
region which is pretty close to what runtime/trace.Span currently
refers to. So, replace that.
https://software.intel.com/en-us/itc-user-and-reference-guide-defining-and-recording-functions-or-regions

This CL also tweaks APIs a bit based on jbd and heschi's comments:

  NewContext -> NewTask
    and it now returns a Task object that exports End method.

  StartSpan -> StartRegion
    and it now returns a Region object that exports End method.

Also, changed WithSpan to WithRegion and it now takes func() with no
context. Another thought is to get rid of WithRegion. It is a nice
concept but in practice, it seems problematic (a lot of code churn,
and polluting stack trace). Already, the tracing concept is very low
level, and we hope this API to be used with great care.

Recommended usage will be
   defer trace.StartRegion(ctx, "someRegion").End()

Left old APIs untouched in this CL. Once the usage of them are cleaned
up, they will be removed in a separate CL.

Change-Id: I73880635e437f3aad51314331a035dd1459b9f3a
Reviewed-on: https://go-review.googlesource.com/108296
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: JBD <jbd@google.com>

src/cmd/trace/main.go

@@ -200,7 +200,7 @@ var templMain = template.Must(template.New("").Parse(`
 <a href="/syscall">Syscall blocking profile</a> (<a href="/syscall?raw=1" download="syscall.profile">⬇</a>)<br>
 <a href="/sched">Scheduler latency profile</a> (<a href="/sche?raw=1" download="sched.profile">⬇</a>)<br>
 <a href="/usertasks">User-defined tasks</a><br>
-<a href="/userspans">User-defined spans</a><br>
+<a href="/userregions">User-defined regions</a><br>
 </body>
 </html>
 `))