Integer type policy

Integers are for numbers, enabling arithmetic like subtractions and for
loops without getting shot in the foot. Unsigneds are for bitfields.

- "int" for numbers that will always be laughably smaller than four
  billion, and where we don't care about the serialization format.

- "int32" for numbers that will always be laughably smaller than four
  billion, and will be serialized to four bytes.

- "int64" for numbers that may approach four billion or will be
  serialized to eight bytes.

- "uint32" and "uint64" for bitfields, depending on required number of
  bits and serialization format. Likewise "uint8" and "uint16", although
  rare in this project since they don't exist in XDR.

- "int8", "int16" and plain "uint" are almost never useful.

cmd/syncthing/perfstats_unix.go

@@ -43,7 +43,7 @@ func savePerfStats(file string) {
 	var prevTime int64
 	var rusage syscall.Rusage
 	var memstats runtime.MemStats
-	var prevIn, prevOut uint64
+	var prevIn, prevOut int64
 
 	t0 := time.Now()
 	for t := range time.NewTicker(250 * time.Millisecond).C {