task: Implement Extend for JoinSet

[elichai]

Motivation

JoinSet Implements FromIterator it's only natural that it also implements Extend

Solution

Implement Extend for JoinSet

[Darksonn]

Use ! to avoid block:

Suggested change

	/// # #[cfg(not(target_family = "wasm"))]
	/// # {
	/// # #![cfg(not(target_family = "wasm"))]

[martin-g]

it seems wasm-pack does not like it.
cargo test --doc passes OK locally.
I don't find any other usage of /// # #![cfg(...)] in the codebase.

[martin-g]

I reverted the last two commits (git reset --hard) to fix the build.