feat(tvix/eval): Support builtins.readDir

Co-authored-by: Griffin Smith <root@gws.fyi>
Change-Id: I5ff19efbe87d8f571f22ab0480500505afa624c5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6552
Autosubmit: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -371,6 +371,12 @@ impl From<i64> for Value {
     }
 }
 
+impl From<&str> for Value {
+    fn from(val: &str) -> Self {
+        Self::String(val.into())
+    }
+}
+
 fn type_error(expected: &'static str, actual: &Value) -> ErrorKind {
     ErrorKind::TypeError {
         expected,