Passes using multiple types of facts (aka snooping)

Imagine a pass which can use multiple types of facts - for example, a cache locality optimisation pass might want information about aliasing, branch selection frequency, and, optionally, the size of the code in a loop.

This might mean multiple types of fact, as well as the ability to OPTIONALLY use a fact type (i.e. if there is no provider of this type of fact, the pass is still valid, but probably not optimal).

It seems to me that the clearest way to handle this is with Effectful typing: each fact is represented with a separate effect type, and optional effects could be handled by a nondeterministic effect. Something like, e.g.:

```haskell
myPassAnalysis :: (Member AliasFact r, Member BranchPredictionFact r, Member NonDetEff BlockSize r) => Eff r (Fact x f) -> n e x -> Fact x f
```