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/builtins/mod.rs

@@ -476,7 +476,7 @@ mod pure_builtins {
     async fn builtin_filterSource(_co: GenCo, #[lazy] _e: Value) -> Result<Value, ErrorKind> {
         // TODO: implement for nixpkgs compatibility
         Ok(Value::Catchable(CatchableErrorKind::UnimplementedFeature(
-            "filterSource".to_string(),
+            "filterSource".into(),
         )))
     }
 
@@ -695,7 +695,7 @@ mod pure_builtins {
     ) -> Result<Value, ErrorKind> {
         // FIXME: propagate contexts here.
         Ok(Value::Catchable(CatchableErrorKind::UnimplementedFeature(
-            "hashString".to_string(),
+            "hashString".into(),
         )))
     }
 
@@ -1392,7 +1392,7 @@ mod pure_builtins {
         // TODO(sterni): coerces to string
         // We do not care about the context here explicitly.
         Ok(Value::Catchable(CatchableErrorKind::Throw(
-            message.to_contextful_str()?.to_string(),
+            message.to_contextful_str()?.to_string().into(),
         )))
     }
 
@@ -1531,9 +1531,7 @@ pub fn pure_builtins() -> Vec<(&'static str, Value)> {
     // TODO: implement for nixpkgs compatibility
     result.push((
         "__curPos",
-        Value::Catchable(CatchableErrorKind::UnimplementedFeature(
-            "__curPos".to_string(),
-        )),
+        Value::Catchable(CatchableErrorKind::UnimplementedFeature("__curPos".into())),
     ));
 
     result