Interleaving of multiple passes

There should be a combinator to take an arbitrary list of passes, and interleave them into a single pass where the fact type is a type-level list of other fact types:

```haskell
interleavePasses :: [forall f. FwdPass m n f] -> FwdPass m n '[]
```

This seems quite Effective (pun most certainly intended).

Start by locating the FwdPass definition and existing pass combinators in the repository, then compare their behavior with the proposed interleavePasses type signature. Clarify how an arbitrary list of passes and the type-level fact list should be represented, and define completion as a working combinator with coverage for interleaving multiple passes.

Written by the indexing model from the issue text.