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.
... in function signatures.
These breaking changes aim at improving usability by allowing users
to take advantage of Rust error management features sur as `ok_or`,
`map_err`, `expect` and the `?` operator. See the `examples` and
`tutorials` to get an idea of the impacts.
