Unnecessary memory allocations in restrict_indices

Hello,

I found this when I was puzzled by a [discourse question](https://discourse.julialang.org/t/strange-memory-allocation-with-distributedarrays/38325).

The function `restrict_indices` (called on line 824 in darray.jl, for example, couldn't figure out how to reference here), allocates an array of indices as big as the array that that you are trying to copy. Thus doubling the memory allocations.

Not sure if there is a way around it (just allocating tuples of ranges of indices or the like), but I thought I should file an issue so it doesn't get lost.

Cheers!