Extra DSignal functionality

[blaxill]

This adds DSignal functions that shadow Signal corresponding functions (but doesn't export them in the prelude of course):

• a new DSignal based Bundle variant- the motivation being that working with the Signal version with DSignal types requires a lot of toSignal coercion.
  • 'delay' for DSignal (e.g. 'delayed' with undefined initial value, that doesn't require explicit reset)
  • 'pipelineTreeFold' which provides a tree structured fold that inserts a flip-flop after every fold operation.

There are alternatives to the DSignal Bundle such as merging with the existing class (which I'm not sure can be done without breaking downstream work), a prelude exported non-shadowing name or perhaps keeping this as a separate package.

[blaxill]

Is 0.99 considered frozen, should I be submitting this to master?

[blaxill]

Closing in favor of #157