feat(tvix/eval): implement string concatenation

Change-Id: If61066e59232b2bad42b5cb5f0f2d9b9c416be8b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6137
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/value/string.rs

@@ -63,6 +63,12 @@ impl NixString {
             Cow::Owned(s) => Cow::Owned(format!("\"{}\"", s)),
         }
     }
+
+    pub fn concat(&self, other: &Self) -> Self {
+        let mut s = self.as_str().to_owned();
+        s.push_str(other.as_str());
+        NixString::Heap(s)
+    }
 }
 
 fn nix_escape_char(ch: char) -> Option<&'static str> {