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/eval/src/tests/mod.rs

@@ -1,4 +1,4 @@
-use crate::value::Value;
+use crate::{value::Value, EvalIO};
 use builtin_macros::builtins;
 use pretty_assertions::assert_eq;
 use rstest::rstest;
@@ -119,7 +119,7 @@ fn identity(#[files("src/tests/tvix_tests/identity-*.nix")] code_path: PathBuf)
 
     let eval = crate::Evaluation {
         strict: true,
-        io_handle: Box::new(crate::StdIO),
+        io_handle: Box::new(crate::StdIO) as Box<dyn EvalIO>,
         ..Default::default()
     };