feat(tvix/eval): construct attribute sets lazily

This thunks the construction of attribute sets. Because Tvix does not
currently have a "strict output" mode, a test had to be disabled that
now displays a thunk representation.

The test will be re-enabled once that is available.

Change-Id: I360332be64cd5c154f9caea21828f6f1b37a265c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6363
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

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

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

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

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