message: Update tails design: minor changes in indy-crypto.
diff: @@ -26,7 +26,7 @@ It's an ordered sequence of elements used for non-revocation proof. "As is" tail
  The new one version of API should return public part of revocation registry without Tails.
  And also it returns `RevocationTailsGenerator` object which should be used to generate all tails one by one and store it somehow
  * `count() -> u32` count of rest Tails in the generator
  - * `next() -> Result<Tail, IndyCryptoError>` - generate next tail
  + * `next() -> Result<Option<Tail>, IndyCryptoError>` - generate next tail
  * Tail::from_bytes
  * Tail::to_bytes
  @@ -35,10 +35,10 @@ It's an ordered sequence of elements used for non-revocation proof. "As is" tail
  * tails_reader_verify_consistency(type, URI, hash) -> bool
  * tails_reader_open(type, URI, hash) -> handle
  * tails_reader_close(handle)
  -* tails_reader_calc_tail_offset()
  +* tails_reader_calc_tail_offset(idx) -> offset
  * tails_reader_tail_from_bytes(tail_bytes)
  * tails_reader_register_type(type,
  - (access_tail)(handle, idx) -> Tail
  + (access_tail)(handle, idx) -> Tail,
  (verify_consistency)(URI) -> hash
  )
  @@ -52,6 +52,3 @@ It's an ordered sequence of elements used for non-revocation proof. "As is" tail
  )
  `config_handle` will be used in `indy_issuer_create_and_store_revoc_reg` (multiply times)
  -
  -## Default VS implementation in SDK
  -TODO
  