refactor(tvix/value): replace static representation with SmolStr

The only uses of the static variant were for `"name"` and `"value"`,
which are both small enough to fit into a SmolStr. The size of
NixString accomodates `String` anyways, so we may as well inline them.

Additionally smol_str is already in the dependency graph because rnix
uses it, and using it for representations of identifiers is sensible.

Change-Id: I9969312256d1657d69128e54c47dc7294a18ce58
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6165
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/value/attrs.rs

@@ -38,8 +38,8 @@ impl AttrsRep {
 
             AttrsRep::KV { name, value } => {
                 *self = AttrsRep::Map(BTreeMap::from([
-                    ("name".into(), std::mem::replace(name, Value::Blackhole)),
-                    ("value".into(), std::mem::replace(value, Value::Blackhole)),
+                    (NixString::NAME, std::mem::replace(name, Value::Blackhole)),
+                    (NixString::VALUE, std::mem::replace(value, Value::Blackhole)),
                 ]));
                 self.map_mut()
             }
@@ -62,7 +62,7 @@ impl AttrsRep {
                 None
             }
 
-            AttrsRep::Map(map) => map.get(&key.to_string().into()),
+            AttrsRep::Map(map) => map.get(&key.into()),
         }
     }
 }