refactor(tvix/eval): let OpCoerceToString select the CoercionKind

Change-Id: I92d58ef216d7e0766af70f019b3dcd445284a95d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10344
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -161,7 +161,7 @@ macro_rules! gen_is {
 }
 
 /// Describes what input types are allowed when coercing a `Value` to a string
-#[derive(Clone, Copy, PartialEq, Debug)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
 pub struct CoercionKind {
     /// If false only coerce already "stringly" types like strings and paths, but
     /// also coerce sets that have a `__toString` attribute. In Tvix, this is