Way back in 538a767e I added `RpcSystem::setFlowLimit()`, a blunt mechanism by which an RPC node can arrange to stop reading new messages from the connection when too many incoming calls are in-flight. This was needed to deal with buggy Sandstorm apps that would stream multi-gigabyte files by doing a zillion writes without waiting, which would then all be queued in the HTTP gateway, causing it to run out of memory.

In implementing that, I inadertently caused the RPC system to do a tree walk on every call message it received, in order to sum up the message size. This is silly, becaues it's much cheaper to sum up the segment sizes. In fact, in the case of a malicious peer, the tree walk is potentially insufficient, because it doesn't count holes in the segments. The tree walk also means that any invalid pointers in the message cause an exception to be thrown even if that pointer is never accessed by the app, which isn't the usual behavior.

I seem to recall this issue coming up in discussion once in the past, but I couldn't find the thread.

For the new streaming feature, we'll be paying attention to the size of outgoing messages. Again, here, it would be nice to compute this size by summing segments without doing a tree walk.

So, this commit adds `sizeInWords()` methods that do this.