Usage of `assert`s

There are a few places in KES implementation where `Control.Exception.assert` is being used. This has no affect since `assert`s will never get triggered, because they are removed by the compiler, unless compiled with either `-O0` or `-fno-ignore-asserts`, which is never the case in this repo.

So, we need figure out whats the purpose of those asserts and either remove them in favor of some other mode of failure or make them count with a cabal flag or something. A good example on how latter can be achieved is [in the consensus repo](https://github.com/input-output-hk/ouroboros-network/blob/c2441629b7bea05eb89a36ee69c6735ee5ae15ff/ouroboros-consensus-cardano/ouroboros-consensus-cardano.cabal#L73-L74).