This code rework the examples to use the new 2018 edition and also
rework the code to avoid using unnecessary 'extern crate' calls.

The 'use extern crate gstreamer as gst', as well as the other gstramer
related crates, were kept, otherwise we'd need to do it on 'Cargo.toml'
but it would make it more difficult to figure out the respective crate
name.