Also add a bunch of casts where needed. I've tried to model everything
in such a way that it minimises casts. The casts *should* be safe, but
it's not always obvious. In the obvious cases, we should have a linter
that validates it. In the non-obvious cases, that linter should warn and
require that we add a null check. I've added some null checks in some
cases but not all.
Also, refactored some of the constructor functions to never assign a
maybe-null value to a non-null struct member, instead using a temporary
local variable to check if construction/allocation succeeded.
This mainly saves spam in test logs, but may save some packets here and
there, if nodes are randomly selected twice for GET_NODES and onion
routing packets.
iphydf