Rust rewrite, initial part

[ku1ik]

This is the initial work for the upcoming 3.0 release, which will be based on full rewrite in Rust.

I've contemplated this, but I didn't have as many good reasons to justify it in the past. However, there's plenty of good reasons for me to do a Rust rewrite now.

Amongst other things the rewrite will:

• allow easy integration of avt (which is written in Rust) to implement things like conversion to pure text, and give asciinema insight into what is actually happening in a terminal, enabling many more features
• allow easy integration of tst (which is written in Rust) to implement terminal live streaming
• allow easy integration of agg (which is written in Rust) so one doesn't need to install a separate tool
• make startup faster - desired when automatically recording all shells/terminal tabs

Last but not least, I enjoy working with Rust much much more than with Python :)

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More about the rewrite: https://discourse.asciinema.org/t/rust-rewrite-of-the-asciinema-cli/777

How to build/test the new version: https://discourse.asciinema.org/t/testing-the-new-rust-version-of-the-asciinema-cli/778/10

The plan for 3.0: https://discourse.asciinema.org/t/the-plan-for-the-asciinema-cli-3-0/790