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.
Also started teaching it about toxcore's alloc/dealloc functions in
hopes of it catching some errors (it doesn't seem to be very good at
this, but maybe better than nothing?).
Currently: 1) libsodium and 2) nacl.
Note that the "nacl" variant is actually libsodium. We just want to make
sure the static analysers see the `VANILLA_NACL` code paths.
