This patch enables more expressive ways to slice and iterate over
the `Id`s in an `IdSet` with `IdSet::iter_in`, which takes an arbitrary
`IdSetSlicer`.
`IdSet::iter_in` efficiently slices out unintended inner parts and
then, within the remaining parts, skips inactive bits by using
`BitSlice::iter_ones` from the `bitvec` crate.
It also delivers several implementations of `IdSetSlicer` so OSTD
consumers can represent `Id` ranges ergonomically.
In the Asterinas kernel, `CpuSet::iter_in` enables a cleaner way to
define an interator that cycles over the CPUs.
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