SmolStrBuilder is less efficient than String for len > 23

[alexheretic]

SmolStrBuilder will allocate at least twice when building a len > 23 SmolStr.

1. When upgrading SmolStrBuilder(Inline) -> SmolStrBuilder(Heap).
2. On SmolStrBuilder::finish when constructing SmolStr(Heap) (because String -> Arc<str> requires this).

Ideally it would only allocate once in this case as with using String.