Turn MockPraos into a real Praos implementation

As part of input-output-hk/ouroboros-network#1652, we'll move the current mock Praos implementation to a separate library related to mock blocks. Eventually, we should promote this mock/prototype implementation to a real implementation so that other blockchains could use it.

Note that Shelley will be using Transitional Praos (TPraos). It would be nice if TPraos could reuse parts of Praos or that we could test TPraos against Praos.