This removes the bulk of our reliance on the rand crate in non-test
envs, paving a way towards a syscall-less rust-lightning and WASM.
Since this is a breaking change for full_stack_target (and several
fuzz targets), go ahead and make other changes to make things more
distinct.
Sadly none of them are really any good without some hand-holding to
generate good input data, and unless sha256 gets stubbed out I'm
not sure they're gonna get good without a symbolic execution fuzzer