Refactor PerfStepTimer to automatically stop on destruct

This eliminates the need to remember to call PERF_TIMER_STOP when a section has
been timed. This allows more useful design with the perf timers and enables
possible return value optimizations. Simplistic example:

```c++
class Foo {
  public:
    Foo(int v) : m_v(v);
  private:
    int m_v;
}

Foo makeFrobbedFoo(int *errno)
{
  *errno = 0;
  return Foo(1);
}

Foo bar(int *errno)
{
  PERF_TIMER_INIT(some_timer);

  return makeFrobbedFoo(errno);
}

int main(int argc, char[] argv)
{
  Foo f;
  int errno;

  f = bar(&errno);

  if (errno)
    return -1;
  return 0;
}
```

After bar() is called, perf_context.some_timer would be incremented as if
Stop(&perf_context.some_timer) was called at the end, and the compiler is still
able to produce optimizations on the return value from makeFrobbedFoo() through
to main().
Don't you need to put this in separate scope with file->Read()? We only want to measure Read() time.