Remove `NodeState::{Waiting,Done}`.

`NodeState` has two states, `Success` and `Done`, that are only used
within `ObligationForest` methods. This commit removes them, and renames
the existing `Waiting` state as `Success`.

We are left with three states: `Pending`, `Success`, and `Error`.
`Success` is augmented with a new `WaitingState`, which indicates when
(if ever) it was last waiting on one or more `Pending` nodes. This
notion of "when" requires adding a "process generation" to
`ObligationForest`; it is incremented on each call to
`process_obligtions`.

This commit is a performance win.

- Most of the benefit comes from `mark_as_waiting` (which the commit
  renames as `mark_still_waiting_nodes`). This function used to do two
  things: (a) change all `Waiting` nodes to `Success`, and (b) mark all
  nodes that depend on a pending node as `Waiting`. In practice, many
  nodes went from `Waiting` to `Success` and then immediately back to
  `Waiting`. The use of generations lets us skip step (a).

- A smaller benefit comes from not having to change nodes to the `Done`
  state in `process_cycles`.

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