Use tx.clone() instead of mpsc::Sender::clone(&tx)

[joekyo]

Both forms are same, but tx.clone() is more concise.

[steveklabnik]

I believe that this is due to a clippy lint: https://rust-lang.github.io/rust-clippy/master/#clone_on_ref_ptr

@carols10cents , opinions?

[carols10cents]

Yep, this style is deliberate to show that this call of clone is cheap. See d0abf1f and #729. We explain this in Chapter 15 (search for "We could have called a.clone() rather than Rc::clone(&a)"). Thanks!