Add GraphBuilder

This provides a more readable API to create graphs in tests. For example:

let mut builder = GraphBuilder::default();
let rev0 = builder
    .node(swhid!(swh:1:rev:0000000000000000000000000000000000000000))?
    .author_timestamp(1708451441, 0)
    .committer_timestamp(1708451441, 0)
    .done();
let rev1 = builder
    .node(swhid!(swh:1:rev:0000000000000000000000000000000000000001))?
    .author_timestamp(1708453970, 0)
    .committer_timestamp(1708453970, 0)
    .done();
let dir2 = builder
    .node(swhid!(swh:1:dir:0000000000000000000000000000000000000002))?
    .done();
let dir3 = builder
    .node(swhid!(swh:1:dir:0000000000000000000000000000000000000003))?
    .done();
let cnt4 = builder
    .node(swhid!(swh:1:cnt:0000000000000000000000000000000000000004))?
    .done();
builder.arc(rev0, dir2, None);
builder.arc(rev1, dir3, None);
builder.arc(dir2, cnt4, None);
builder.arc(dir3, cnt4, None);

let graph = builder.done()?;

instead of:

let backward_graph = SwhUnidirectionalGraph::from_underlying_graph(
    PathBuf::new(),
    Left(VecGraph::from_arc_list(vec![
        (2, 0),
        (3, 1),
        (4, 2),
        (4, 3),
    ])),
)
.init_properties()
.load_properties(|properties| {
    Ok(properties
        .with_maps(VecMaps::new(vec![
            swhid!(swh:1:rev:0000000000000000000000000000000000000000),
            swhid!(swh:1:rev:0000000000000000000000000000000000000001),
            swhid!(swh:1:dir:0000000000000000000000000000000000000002),
            swhid!(swh:1:dir:0000000000000000000000000000000000000003),
            swhid!(swh:1:cnt:0000000000000000000000000000000000000004),
        ]))
        .unwrap()
        .with_timestamps(
            VecTimestamps::new(vec![
                (Some(1708451441), Some(0), Some(1708451441), Some(0)),
                (Some(1708453970), Some(0), Some(1708453970), Some(0)),
                (None, None, None, None),
                (None, None, None, None),
                (None, None, None, None),
                (None, None, None, None),
            ])
            .unwrap(),
        )
        .unwrap())
})
.unwrap();
let graph = Transposed(&backward_graph);

Depends on !401 (merged)