make Block cheaply cloneable

Since Block appears in APIs over multiple crates (mostly as `&Block`) and it doesn’t offer mutation methods, it would be reasonable to assume that an implementation of a consumer should want to hold on to the bytes it got — which today requires making a copy. The only adversely affected method would be `Block::into_inner()`, which probably would become less useful by such a change, as moving between Arc and Vec always requires an allocation.

What do you think?