Incremental routing code refactoring (1)

[jerry73204]

This PR pushes a series of internal code refactoring based on Oliver's routing-refactor branch.

• Replace full qualified paths with "use"s. For example, std::borrow::Cow is rewritten as Cow for readability.
• Some macros are changed to functions. For example, send_to_first and send_to_many`. It reduces the macro processing a bit.
• Use NodeId, FaceId and NodeIndex instead of usize for index numbers. It improves readability and avoids accidental misusage.
• The Clone impls are removed from the types that are not intended to be cloned, such as Face. They are wrapped to Arc instead.
• Some function generics are removed from internal code, such as Face: Borrow<Arc<FaceState>>.
• The type of the field Node::links is changed from Vec to HashSet to speed up .contains() search.

More incremental changes will be pushed later. Before it happens, we'd make sure these changes are correctly done.

[wyfo]

@OlivierHecart is this PR completely outdated so we can close it?