Add caching to `DataExpressionSource`

[jvdb]

Generally we try very hard to avoid mutable state in Metal, but in the case of DataExpressionSource this causes serious performance problems. IO laziness when parsing data with a DataExpressionSource means that for evaluating every Def or Until, the entire expression is evaluated. This has led to a slowdown of a factor of more than 100x in a benchmark.

So in this specific case, it makes sense for DataExpressionSource to cache its result.