VecExt added a .map() convenience method on Vec<T> that simply called
.into_iter().map(f).collect(). This thin wrapper provided no
optimisation benefit and obscured a standard iterator pattern behind a
nightly feature gate and an extra dependency.
Replaced the single call site in exo_pyo3_bindings with the equivalent
iterator chain and removed the ext module, the extend dependency, and
the trait_alias feature gate from the util crate.
Test plan:
- CI
