refactor(tvix/eval): move NumberPair struct definition to vm module

This isn't relevant to the value type itself.

Change-Id: I678bc92a8a530b1081ed498bf3ff7925217bcc01
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6081
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/eval.rs

@@ -1,7 +1,7 @@
 use rnix::{self, types::TypedNode};
 use std::fmt::Write;
 
-use crate::errors::{Error, EvalResult};
+use crate::errors::EvalResult;
 
 pub fn interpret(code: String) -> EvalResult<String> {
     let ast = rnix::parse(&code);