Simplify CSPRNG API

[itzmeanjan]

RandomSHAKE CSPRNG allows one to pass required bit-security level, which decides how long the seed should be to initialize the underlying XOF. But after conversation with people who know this better than I do, I'm convinced a break change, removing the need to specify bit-security level is worth. Instead the seed required to initialize the underlying XOF is longer now. It should be RATE-bits wide s.t. RATE is the bit-width of the rate portion of the keccak sponge of the XOF chosen. For example, with SHAKE256 XOF, the seed required to initialize the CSPRNG is now 136-bytes as rate of SHAKE256 is 1088-bits. It simplifies the API and lets user pass longer seed - hopefully more entropy 😉

Execute it.