Gmoshkin/fix fiber detach

Summary

• feat: fiber::Builder::{start_non_joinable, defer_non_joinable}

• fix: remove JoinHandle::{detach, detach_checked, detach_and_forget} as they're unsafe actually

• chore: doc-comment tweaks

• refactor: don't use crate::Result, use crate::Result instead

• feat: reserved fiber id constants

• chore: cleanup module imports and declarations in fiber.rs

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• Version is bumped in the appropriate Cargo.toml files