remove `usize: DepGraphRead` and add `Untracked`

remove `usize: DepGraphRead` and add `Untracked`

The idea is that a `usize` is sort of ambiguous: in this case, it
represents indices that do not need tracking, but it could as easily be
some data read out from a tracked location, and hence represent tracked
data. Therefore, we add an `Untracked` type that lets user assert
that value is not tracked.

Also correct various typos.