Compatibility with Rust 2021

[gicrisf]

First of all, thanks for this library: I didn't really need it for anything, but I tried it for fun and it works like a charm.
The compiler, though, complains with warnings about how the trait are declared.
As said in the official docs

The use of the dyn keyword to identify trait objects will be mandatory in Rust 2021.

So I updated all the files involved. Moreover, I updated the examples with actually working suggestions.
As you can see, those are humble editings and don't mess with the actual code in any way.