runtime: delete UpdateMemStats, replace with ReadMemStats(&stats).

Unexports runtime.MemStats and rename MemStatsType to MemStats.
The new accessor requires passing a pointer to a user-allocated
MemStats structure.

Fixes #2572.

R=bradfitz, rsc, bradfitz, gustavo
CC=golang-dev, remy
https://golang.org/cl/5616072

src/pkg/runtime/gc_test.go

@@ -10,20 +10,21 @@ import (
 )
 
 func TestGcSys(t *testing.T) {
+	memstats := new(runtime.MemStats)
 	runtime.GC()
-	runtime.UpdateMemStats()
-	sys := runtime.MemStats.Sys
+	runtime.ReadMemStats(memstats)
+	sys := memstats.Sys
 
 	for i := 0; i < 1000000; i++ {
 		workthegc()
 	}
 
 	// Should only be using a few MB.
-	runtime.UpdateMemStats()
-	if sys > runtime.MemStats.Sys {
+	runtime.ReadMemStats(memstats)
+	if sys > memstats.Sys {
 		sys = 0
 	} else {
-		sys = runtime.MemStats.Sys - sys
+		sys = memstats.Sys - sys
 	}
 	t.Logf("used %d extra bytes", sys)
 	if sys > 4<<20 {