Be able to functorize docteur over a Mirage_block.S

[dinosaure]

Currently, docteur implements its own block device. It's highly related to mirage/mirage-block#53. Indeed, we must have a mmap syscall which is outside the Lwt monad. If it's the case, we can imagine then a functorization of docteur_{solo5,unix} over a Mirage_block.S (and combine it with mirage-block-ccm for instance).