rustdoc: Refactor `get_all_types` so `args` is fully shared between loop iterations

[camelid]

1. Refactor add_generics_and_bounds_as_types so it handles all types, not just type parameters.
2. Remove the !args.is_empty() conditional in get_fn_inputs_and_outputs.
3. Extend all_types with args at the end of the loop.

cc @GuillaumeGomez

[camelid]

Assigning myself for now since I may look into it.

[JmPotato]

@rustbot claim