Remove `get_` from getters

The Rust-idiomatic way of naming getters is to use the attribute name only. We would write:

let seqnum = segment.seqnum();
if let Some(structure) = event.structure() {
    ...
}

instead of:

let seqnum = segment.get_seqnum();
if let Some(structure) = event.get_structure() {
    ...
}

See !633 (comment 698679).