Implement yield $x

[01mf02]

This implements yield $x, as a complement to break $x.

This is WIP to address ynqa/jnv#114.

$ cargo run -q -- -n 'label $x | (((1, (2 | yield $x)) | . - 1))'
0
2
$ cargo run -q -- -n 'label $x | ((1, (2 | yield $x), 3, (4 | yield $x)) | empty)'
2
4