Fix `mapchunks` and write Test for it

[schlichtanders]

While looking for a way to flatten an DArray, I found mapchunks, which looks perfectly suited for this job. Unfortunately it is currently buggy. Probably because there is no test for it.

Dagger.jl/src/array/darray.jl

Lines 387 to 393 in cec17a1

	function mapchunks(f, d::DArray{T,N,F}) where {T,N,F}
	chunks = map(d.chunks) do chunk
	owner = get_parent(chunk.processor).pid
	remotecall_fetch(mapchunk, owner, f, chunk)
	end
	DArray{T,N,F}(d.domain, d.subdomains, chunks, d.concat)
	end

The DArray constructor used here does not exist any longer. There is a new argument - the partitioning - which need to be added.

- DArray{T,N,F}(d.domain, d.subdomains, chunks, d.concat) 
+ DArray{T,N}(d.domain, d.subdomains, chunks, d.partitioning, d.concat) 

[schlichtanders]

Furthermore, it would be great if

• mapchunks also supports EagerThunk
• mapchunks handles the new DArray type T correctly. (By inspecting the mapped chunks, or falling back to type-inference or Any, if there are no chunks)