chore(tvix/eval): remove From<SmolStr> for NixString instance

No longer needed, and in some cases caused some extra work.

Change-Id: I64e8e7292573bdc92a9c7a8e470e33f8c526f311
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8152
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/impure.rs

@@ -1,6 +1,5 @@
 use builtin_macros::builtins;
 use genawaiter::rc::Gen;
-use smol_str::SmolStr;
 
 use std::{
     env,
@@ -12,7 +11,7 @@ use crate::{
     io::FileType,
     value::NixAttrs,
     vm::generators::{self, GenCo},
-    Value,
+    NixString, Value,
 };
 
 #[builtins]
@@ -38,14 +37,14 @@ mod impure_builtins {
         let dir = generators::request_read_dir(&co, path).await;
         let res = dir.into_iter().map(|(name, ftype)| {
             (
-                name,
+                NixString::from(name.as_str()),
                 Value::String(
-                    SmolStr::new(match ftype {
+                    match ftype {
                         FileType::Directory => "directory",
                         FileType::Regular => "regular",
                         FileType::Symlink => "symlink",
                         FileType::Unknown => "unknown",
-                    })
+                    }
                     .into(),
                 ),
             )