Check for ByteString memory leaks

[edsko]

Since ByteString's can point to (larger) bytestrings, it is easy to create "bytestring memory leaks". We have already fixed some of these, but there may be others.

[edsko]

We store bytestrings primarily in two locations: As PIDs and as undecoded messages. We now copy the bytestring in both those places, and moreover make sure that this copy happens eagerly. In addition, messages put into type channels are now decoded eagerly, so we don't retain bytestrings there at all.

This doesn't mean this issue is definitely completely fixed, but I'll close it until further evidence appears that there are additional leaks.