feat(tvix/eval): Implement builtins.elem

Change-Id: Id99c1d33f87ad9866990d3483d3531e9e48f861f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6916
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -359,6 +359,12 @@ impl Display for Value {
     }
 }
 
+impl From<bool> for Value {
+    fn from(b: bool) -> Self {
+        Value::Bool(b)
+    }
+}
+
 impl From<i64> for Value {
     fn from(i: i64) -> Self {
         Self::Integer(i)