refactor(tvix/eval): remove Error::InvalidKeyType

We're confident that we're handling all branches that can reasonably
occur from valid AST, any other cases should be considered a critical
evaluator bug and panic rather than surfacing something that looks
like user error.

Change-Id: If96966eb32b8ff12fcaeb9ea3b0c8fc51b6abd11
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6205
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/value/attrs.rs

@@ -256,11 +256,7 @@ impl NixAttrs {
                     continue;
                 }
 
-                other => {
-                    return Err(Error::InvalidKeyType {
-                        given: other.type_of(),
-                    })
-                }
+                other => panic!("unexpected attribute key type: {}", other.type_of()),
             }
         }