Add overload with consts to Expression walk for read-only usage

The existing Expression walk always operates with visit methods that take 'Expression*' and allows replacement of the current Expression node. We should add an overload which accepts walkers with 'const Expression&' visit methods and disallows node replacement for read-only use cases.

When we work on this ticket and modify the entrypoint to the walk function we should consider changing the mutable walk syntax from

if (auto changed = walk(&walker, root.get()))

    root = changed.release();

to something like

root = walk(&walker, std::move(root));