Add tests for constant-space NAR processing

[imalsogreg]

When working with NARs that are large, we want to test that our serialization/deserialization can work in constant space. This can be achieved with a run of the test suite with RTS parameters limiting the heap size, e.g. ./space-tests +RTS -K100K (taken from http://neilmitchell.blogspot.com/2015/09/detecting-space-leaks.html)

@shlevy Could you add links to this ticket to issues in nix tooling where memory issues affected NAR processing? Those could suggest test fixtures and specific regression tests for us.

[shlevy]

There are a bunch...

NixOS/nix#1988
NixOS/nix#1969
NixOS/nix#619
NixOS/nix#1754
NixOS/nix#1681
NixOS/nix#1400
NixOS/nix#358

and probably more.

[imalsogreg]

Thanks for the examples!

We now have constant-space streaming tests for large files in #7

[imalsogreg]

Closed by #7

[ryantrinkle]

Wow, this is awesome! Thanks guys - just got bit by NixOS/nix#1988

[nh2]

I made a PR that should fix (at least one more part) of that in nix: NixOS/nix#2206