Arrays with Missing Hunks

Is it feasible that DArray could support distributed arrays in which pieces are missing?

For instance, say I have the following:

```
X X X
X X X X
X X
```

where each `X` represents a dense portion of an array and the blanks represent a portion which can be modeled as representing a single value throughout.

In my mind, DArray has some sort of address table and, when it does a lookup, it could note the gap and return an appropriately formatted response using the constant value.

No files, tests, or entry points are named. Begin by reviewing DArray's current representation and lookup behavior, then clarify how missing regions and their constant values should be represented and exposed before determining what completion would mean.

Written by the indexing model from the issue text.