add function to get total live bytes

[JeffBezanson]

The GC has this counter but AFAICT we don't expose it in any way. This can be pretty useful, since it's one of the most accurate ways to see how much memory your program is actually using.

Of course, the only way to determine actually live bytes is to do a full GC right before looking at the counter. That gives the most accurate result, but is slow and also leaves out memory used due to GC not running yet, which could be important in practice. So we need to decide whether to run GC (implemented here), or instead return live_bytes from the last GC plus the number of (possibly garbage) bytes allocated since then.

[JeffBezanson]

From triage: this should probably not call gc(), since that could change memory usage when we're trying to profile it.

[StefanKarpinski]

Perhaps gc could return the total number of live bytes as well and have a guarantee that it's the exact number of live bytes at the point where the collection finishes. Otherwise something could have happened between a call to GC.gc() and a call to GC.live_bytes().