v0.27

[basiliscos]

• [feature] new interface message_visitor_t
• [feature] new interface message_stringifier_t and the default implementation
  default_stringifier_t which allows to dump messages. It is not a production but
  a diagnostic/debug tool, due to performance restrictions.
• [feature] system_context_t provides a reference to default message_stringifier_t;
  it is possible to have a custom one
• [feature, breaking] extended_error_t holds a reference to a request message,
  which caused an error
• [examples, tests, win32] fix ev examples and tests
• [example] modernize examples/thread/sha512.cpp to use recent openssl version
• [breaking] cmake requirements are lowered to 3.15
• [breaking] fix minor compilation warnings

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This discussion was created from the release v0.27.