Split utils::WakerList into WakerVec and WakerArray

[yoshuawuyts]

This works towards #68, but we should have a const-variant of the existing WakerList. Probably one way to do it would be to separate it by name: e.g. WakerVec (dynamic size) vs WakerArray (static size). This would make PRs like #75 faster and not rely on allocations.

[yoshuawuyts]

This should be a fairly straighforward copying of files, renaming some types, and doing a switch from Vec to using const-generics. It doesn't require any async-specific knowledge, and should be fairly straighforward to do!