test(tvix/nix-compat): add debug assertions for nar reader r/8007

Adds debug assertions to ensure that the reader's variants are upheld.
If any of the following happens, then the currently in use reader must
be abandoned:
  * A directory or file reader encounters an error
  * A directory or file reader is dropped before being fully read from
Additionally, a directory reader must not be read from again after it
has returned None.
These checks are only used when debug_assertions are on, so vanish in
release mode.

Resolves two TODO items added by edef

Change-Id: I27bd9643a632798db5351957506c166b9bd5ca4e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11508
Reviewed-by: flokli <flokli@flokli.de>
Reviewed-by: edef <edef@edef.eu>
Autosubmit: Aria Shrimpton <me@aria.rip>
Tested-by: BuildkiteCI

0 files changed, 0 insertions, 0 deletions