refactor(tvix/eval): Box Value::String

NixString is *quite* large - like 80 bytes - because of the extra
capacity value for BString and because of the context. We want to keep
Value small since we're passing it around a lot, so let's box the
NixString inside Value::String to save on some memory, and make cloning
ostensibly a little cheaper

Change-Id: I343c8b4e7f61dc3dcbbaba4382efb3b3e5bbabb2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10729
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/impure.rs

@@ -50,15 +50,12 @@ mod impure_builtins {
                         NixString::from(
                             String::from_utf8(name.to_vec()).expect("parsing file name as string"),
                         ),
-                        Value::String(
-                            match ftype {
-                                FileType::Directory => "directory",
-                                FileType::Regular => "regular",
-                                FileType::Symlink => "symlink",
-                                FileType::Unknown => "unknown",
-                            }
-                            .into(),
-                        ),
+                        Value::from(match ftype {
+                            FileType::Directory => "directory",
+                            FileType::Regular => "regular",
+                            FileType::Symlink => "symlink",
+                            FileType::Unknown => "unknown",
+                        }),
                     )
                 });