SPLIT should also store length

[Lartu]

LDPL vectors give no way to know their length. While I'd love to refactor them and make reading an index before it has been defined ilegal, this would break compatibility with older ldpl versions. So in order to know the length of a split, either the split function should return the length of the split vector or a way to know the lengh of a vector should be added to the language (in a way that makes sense).