Package tracing will provide usage and runtime support for Linux-style
traceevents/tracepoints for Go.

This patch comes with the runtime support to attach/detach probes to/from
trace-events and stop/restart the world so that attaching/detaching can
be done safely while nothing else is running. Having attach/detach under
STW allows regular probe list iteration to be done without locks.

The next patch will add gotrace utility which automatically turns
//trace:event in-source comments into proper trace-event definitions.

Below is excerpt from tracing usage. Please refer to tracing.go for full
text.

---- 8< ----
Package tracing provides usage and runtime support for Go tracing facilities.

Trace events

A Go package can define several events of interest to trace via special
comments. With such definition a tracing event becomes associated with trace
function that is used to signal when the event happens. For example:

	package hello

	//trace:event traceHelloPre(who string)
	//trace:event traceHello(who string)

	func SayHello(who string) {
		traceHelloPre(who)
		fmt.Println("Hello, %s", who)
		traceHello(who)
	}

By default trace function does nothing and has very small overhead.

Probes

However it is possible to attach probing functions to events. A probe, once
attached, is called whenever event is signalled in the context which triggered
the event and pauses original code execution until the probe is finished. It is
possible to attach several probing functions to the same event and dynamically
detach/(re-)attach them at runtime. Attaching/detaching probes must be done
under tracing.Lock. For example:

	type saidHelloT struct {
		who  string
		when time.Time
	}
	saidHello := make(chan saidHelloT)

	tracing.Lock()
	p := traceHello_Attach(nil, func(who string) {
		saidHello <- saidHelloT{who, time.Now()}
	})
	tracing.Unlock()

	go func() {
		for hello := range saidHello {
			fmt.Printf("Said hello to %v @ %v\n", hello.who, hello.when)
		}
	}()

	SayHello("JP")
	SayHello("Kirr")
	SayHello("Varya")

	tracing.Lock()
	p.Detach()
	tracing.Unlock()

	close(saidHello)

For convenience it is possible to keep group of attached probes and detach them
all at once using ProbeGroup:

	pg := &tracing.ProbeGroup{}

	tracing.Lock()
	traceHelloPre_Attach(pg, func(who string) { ... })
	traceHello_Attach(pg, func(who string) { ... })
	tracing.Unlock()

	// some activity

	// when probes needs to be detached (no explicit tracing.Lock needed):
	pg.Done()

Probes is general mechanism which allows various kinds of trace events usage.
Three ways particularly are well-understood and handy:

	- recording events stream
	- profiling
	- synchronous tracing

...