refactor(tvix/eval): generalize EvalIO container

Don't restrict to a Box<dyn EvalIO>.

There's still one or two places where we do restrict, this will be
solved by b/262.

Change-Id: Ic8d927d6ea81fa12d90b1e4352f35ffaafbd1adf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10639
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/serde/src/de.rs

@@ -3,7 +3,7 @@
 use serde::de::value::{MapDeserializer, SeqDeserializer};
 use serde::de::{self, EnumAccess, VariantAccess};
 pub use tvix_eval::Evaluation;
-use tvix_eval::Value;
+use tvix_eval::{EvalIO, Value};
 
 use crate::error::Error;
 
@@ -43,7 +43,7 @@ where
 pub fn from_str_with_config<'code, T, F>(src: &'code str, config: F) -> Result<T, Error>
 where
     T: serde::Deserialize<'code>,
-    F: FnOnce(&mut Evaluation),
+    F: FnOnce(&mut Evaluation<Box<dyn EvalIO>>),
 {
     // First step is to evaluate the Nix code ...
     let mut eval = Evaluation::default();