feat(tvix/vm): implement first nested attribute set construction

This can construct non-overlapping nested attribute sets (i.e. `{ a.b
= 1; b.c = 2; }`, but not `{ a.b = 1; a.c = 2; }`).

In order to do the latter, it's necessary to gain the ability to
manipulate the in-progress attribute set construction. There's
multiple different options for this ...

Change-Id: If1a762a720b175e8eb4216cbf96a7434d22640fb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6106
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/errors.rs

@@ -6,6 +6,10 @@ pub enum Error {
         key: String,
     },
 
+    InvalidKeyType {
+        given: &'static str,
+    },
+
     TypeError {
         expected: &'static str,
         actual: &'static str,