IndexEntry: enhance! Also, R# is fun.

1. Eliminate one-off `IComparer<T>` by implementing `IComparable<T>` on `IndexEntry`.
2. Micro-optimization: if we know how big a `List<T>` will be, we might as well use it.
3. With conflicts, `IndexEntry.StageLevel` is useful in `DebuggerDisplay`.

I also picked a few R# suggestions to clear out project-wide...things we're mostly consistently on. Take them or leave them.