External arrays
===============

It would be nice to support "external arrays", i.e. arrays that aren't stored
"directly" in memory.

Use cases:
* Compressed byte array
* Array on GPU
* Array on remote system
* Array in compressed file
* Array in uncompressed file ("soft mmap")

Problems:
* When operating on the array, then what arena should be used?
  If the array can read/write from disk, then a plain "allocation-only"
  arena won't be enough.