fix(tvix/eval): Replace inner NixString repr with Box<Bstr>

Storing a full BString here incurs the extra overhead of the capacity
for the inner byte-vector, which we basically never use as Nix strings
are immutable (and we don't do any mutation / sharing analysis).
Switching to a Box<BStr> cuts us from 72 bytes to 64 bytes per
string (and there are a lot of strings!)

Change-Id: I11f34c14a08fa02759f260b1c78b2a2b981714e4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10794
Autosubmit: aspen <root@gws.fyi>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/vm/mod.rs

@@ -558,7 +558,7 @@ where
                                 return frame.error(
                                     self,
                                     ErrorKind::AttributeNotFound {
-                                        name: (**key).clone().into_string_lossy()
+                                        name: key.to_str_lossy().into_owned()
                                     },
                                 );
                             }