refactor(tvix/eval): Box the strings in CatchableErrorKind

These strings are allocated once and never changed, so they don't need
the additional overhead of a capacity given by String - instead, we can
use Box<str> and save on 16 bytes for each of these, *and* for each
Value since this is currently the largest Value variant.

Change-Id: I3e5cb070fe6c5bf82114c92d04f6bae775663a7e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10796
Autosubmit: aspen <root@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -40,11 +40,11 @@ use crate::{SourceCode, Value};
 ///
 #[derive(Clone, Debug)]
 pub enum CatchableErrorKind {
-    Throw(String),
+    Throw(Box<str>),
     AssertionFailed,
-    UnimplementedFeature(String),
+    UnimplementedFeature(Box<str>),
     /// Resolving a user-supplied angle brackets path literal failed in some way.
-    NixPathResolution(String),
+    NixPathResolution(Box<str>),
 }
 
 #[derive(Clone, Debug)]