feat(tvix/eval): semi-strictly evaluate output values of the VM

This essentially makes the VM behave like `nix-instantiate --eval
--strict`, i.e. data structures are traversed strictly and thunks are
forced. Thunks embedded in closures are not forced.

This allows us to re-enable tests that were disabled because they
needed to output nested thunk contents, but is overall a behaviour
that must be configurable later on, as it is not cmopatible with e.g.
an evaluation of nixpkgs.

Change-Id: I5303a5c8e4322feab1384fdb7712fecb950afca5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6372
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/disabled-identity-nested-attrs.nix

@@ -1,3 +0,0 @@
-# TODO: temporarily disabled because need "strict output" (b is
-# thunked)
-{ a = { b = null; }; }

tvix/eval/src/tests/tvix_tests/identity-nested-attrs.nix

@@ -0,0 +1 @@
+{ a = { b = null; }; }