Concurrency feature

[yhara]

Concurrency support is an essential feature in modern programming languages. However, there are various styles available: native threads (such as Ruby's Thread), stackless coroutines (using async/await), and stackful coroutines (like Ruby's Fiber).

Recently, I had a discussion with the author of pen-lang and was quite impressed by the techniques used to implement stackful coroutines that (optionally) works on top of Tokio. Now, I'm contemplating whether Shiika could implement a similar feature — perhaps calling it Shiika's coroutine, or maybe even "Shiroutine"? 😂

[yhara]

Moved to #545.