feat(tvix/eval): Box Value::Catchable

This is now the only enum variant for Value that is larger than 8
bytes (it's 16 bytes), so boxing it (especially since it's not
perf-critical) allows us to get the Value size down to only 16 bytes!

Change-Id: I98598e2b762944448bef982e8ff7da6d6683c4aa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10798
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Autosubmit: aspen <root@gws.fyi>

tvix/eval/src/value/attrs.rs

@@ -375,7 +375,7 @@ impl NixAttrs {
                     continue;
                 }
 
-                Value::Catchable(err) => return Ok(Err(err)),
+                Value::Catchable(err) => return Ok(Err(*err)),
 
                 other => return Err(ErrorKind::InvalidAttributeName(other)),
             }