There are spots in ParallelArray.js where we have for-loops that copy substrings (i.e. contiguous subsequences) from one array to another. That should be an intrinsic operation (that we make super-fast).
1. I meant that these are copying substrings from one *dense* array to another *dense* array. 2. We would want the intrinsic to work on Typed Arrays as well. (That or have a separate intrinsic for the typed array cases, if that ends up being a better strategy.)