feat(tvix/eval): add module for string type implementation

Change-Id: I5e4465acc4a676c10d7374b14f7a09240202b466
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6085
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/value/string.rs

@@ -0,0 +1,13 @@
+use std::fmt::Display;
+
+/// This module implements Nix language strings and their different
+/// backing implementations.
+
+#[derive(Debug, Hash, PartialEq)]
+pub struct NixString(String);
+
+impl Display for NixString {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(self.0.as_str())
+    }
+}