Refactor `rustc_peek`
Refactor `rustc_peek`

We now use `DataflowResultsCursor` to get the dataflow state before
calls to `rustc_peek`. The original version used a custom implementation
that only looked at assignment statements. This also extends
`rustc_peek` to take arguments by-value as well as by-reference, and
allows *all* dataflow analyses, not just those dependent on `MoveData`,
to be inspected.

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust