rust: add num module and Integer trait

rust: add num module and Integer trait

Introduce the `num` module, which will provide numerical extensions and
utilities for the kernel.

For now, introduce the `Integer` trait, which is implemented for all
primitive integer types to provides their core properties to generic
code.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251108-bounded_ints-v4-1-c9342ac7ebd1@nvidia.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>