Fix mapchunks and add tests for it

[alecarraro]

Hi, this PR proposes a fix for (#476). Specifically I implemented the following changes:

1. mapchunks now uses the correct DArray constructor
2. I switched to Dagger.spawn instead of fetch/collect, so mapchunks integrates with Dagger’s scheduler. I also set Options(meta=false) explicitely to highlight that f gets the materialized chunk data but I guess it might not be necessary.
3. For type inference, I added a _mapchunks_eltype helper: it looks at chunktype when available, promotes across chunks, and falls back to Any if type inference is not possible. I think an other option could be to use promote_op(f, chunktype(chunk)), but then it should be documented that it fails if the chunk has heterogeneous types. What do you think?
4. Added some tests. The testset for mapchunks takes 50% longer than the testset for map on my computer. Lmk if you'd prefer it to keep it lighter

Also, the function mapchunks is undocumented at the moment. I would be happy to fix that as well, but it makes more sense to do it after a design is finalized. Since this issue has been around for a while, I guess there might be some prior context I’m missing (i.e planned API changes). Happy to adjust if that's the case!