feat(tvix/eval): implement list concatenation

Change-Id: Icdf715d116371a9f139bdf95266410bf967bef25
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6144
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/opcode.rs

@@ -41,6 +41,7 @@ pub enum OpCode {
 
     // Lists
     OpList(usize),
+    OpConcat,
 
     // Strings
     OpInterpolate(usize),