feat(tvix/eval): implement upvalue resolution in with scopes

These need to be handled specially by the runtime if the compiler
determines that a given local must be resolved via `with`.

Note that this implementation has a bug: It currently allows `with`
inside of nested lambdas to shadow statically known identifiers. This
will be cleaned up in the next commit.

Change-Id: If196b99cbd1a0f2dbb4a40a0e88cdb09a009c6b9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6299
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/value/mod.rs

@@ -80,6 +80,17 @@ impl Value {
         }
     }
 
+    pub fn as_str(&self) -> EvalResult<&str> {
+        match self {
+            Value::String(s) => Ok(s.as_str()),
+            other => Err(ErrorKind::TypeError {
+                expected: "string",
+                actual: other.type_of(),
+            }
+            .into()),
+        }
+    }
+
     pub fn to_string(self) -> EvalResult<NixString> {
         match self {
             Value::String(s) => Ok(s),