feat(tvix/eval): implement attrset update (//) operator

The underlying implementation does a few tricks based on which pair of
attrset representations is encountered.

Particularly the effect of short-circuiting the empty cases might be
relevant in nixpkgs/NixOS, due to the use of lib.optionalAttrs.

Change-Id: I22b978b1c69af12926489a71087c6a6219c012f3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6140
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/opcode.rs

@@ -33,6 +33,7 @@ pub enum OpCode {
     // Attribute sets
     OpAttrs(usize),
     OpAttrPath(usize),
+    OpAttrsUpdate,
 
     // Lists
     OpList(usize),