0.6.0

CHANGES:

• Raise an error when Ssl.flush isn't successful (#104, #120)
• Add an API-compatible Ssl.Runtime_lock module. The functions in this module
  don't release the OCaml runtime lock. While they don't allow other OCaml
  threads to run concurrently, they don't perform any copying in the underlying
  data, leading certain workloads to be faster than their counterparts that
  release the lock. (#106)
• Guarantee Ssl.output_string writes the whole string by retrying the
  operation with unwritten bytes (#103, #116)
• Fix calls in C stubs that need to call ERR_clear_error before the underlying
  OpenSSL call (#118)
• Add a module Ssl.Error to retrieve OpenSSL errors in a structured way (#119)
• Deprecate Ssl.{SSLv23,SSLv3,TLSv1,TLSv1_1}, which were were formally
  deprecated in March 2021 and earlier (#115).