Rework `amount_iterator` in the search generator

[lgarron]

This returns U2' but it should return U2:

cargo run --release -- search --max-depth 2 --generator-moves "U2" --scramble-alg "U2" ./samples/3x3x3/3x3x3-Reid.def.json

It's easy to fix this by adding (TurnMetric::Hand, MoveCount(2) | MoveCount(1)) => vec![r#move.amount], but I'd like to rethink the whole calculation. In particular:

• The existing logic already aimed to do this — what went wrong?
• Should the (TurnMetric::Quantum, …) cases return r#move.amount instead of 1 (and -r#move.amount instead of -1)?