Add some useful functions into the description of the module HASH
Add some useful functions into the description of the module HASH

- null: [= digest_string ""], this value is used to initialise some values
  such as an [Hash.t array] with an impossible value (Git should __never__
  create an Git object with the hash [digest_string ""]
- length: a better name for [digest_size]
- feed: be able to feed a [Bigstringaf.t] value

mirage/ocaml-git,mirage/ocaml-git,mirage/ocaml-git