feat(tvix/eval): implement inherit in attribute set literals

Straightforward implementation, evaluating the elements of an inherit
and preparing the stack so that `OpAttrs` sees all relevant values
when constructing the attribute set itself.

The emitted instructions for inheriting a lot of values from the same
attribute set are inefficient, but it's too early to say whether this
actually matters.

Change-Id: Icb55a20936d4ef77173f34433811c5fa5d2c9ecc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6214
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit-literal.exp

@@ -0,0 +1 @@
+15

tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit-literal.nix

@@ -0,0 +1,2 @@
+# the 'from' part of an `inherit` can be any expression.
+{ inherit ({a = 15;}) a; }.a

tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit.exp

@@ -0,0 +1 @@
+{ a = 15; }

tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit.nix

@@ -0,0 +1,2 @@
+let a = 15;
+in { inherit a; }