Add a convenience function pb_get_encoded_size()

Add a convenience function pb_get_encoded_size()

There is minimal size penalty from this, and it is probably much more
intuitive to use than PB_OSTREAM_SIZING itself.

This has been suggested before also, but I ended up refusing it back
them. Reconsidering it now, I see that an intuitive API is much better
than any amount of documentation explaining a non-intuitive API.

Update issue 16
Status: FixedInGit