Message protocol: Cucumber Messages

[bbros-dev]

Cucumber Messages is a message protocol for representing results and other information from Cucumber - Replaces json and junit formatters.

Stability/Performance:

With Cucumber Messages, several messages containing smaller pieces of information are emitted continuously to a stream,
avoiding high memory consumption and enabling real-time processing of results.

Additional functionality (motivation):

The protocol aims to decouple various components of the Cucumber platform so that:

• Each component only needs to know about a subset of messages
• Gherkin is decoupled from the Cucumber execution component
  • This is part of a strategy to support other formats such as Markdown and Excel

[tyranron]

Moved this to cucumber-rs/cucumber#169