Isosurface performance enhancements

This gives a 6x speed boost with a large volume on my system. The changes:

- Avoid calling `collect` on the whole Dict, which was very expensive because of all the heap-allocated tuples. Instead just iterate over keys to construct `vtD` and collect values to construct `vtAry`.
- Avoid allocating temporary arrays, replacing them with tuples where that simplifies the code.
- Remove expensive `vId` inner function. Unfortunately this results in some ugliness, but I think the performance gain is worth it.