BundleBridge enhancements

[hcook]

This PR adds two new capabilities to the diplomatic BundleBridge API.

BundleBroadcast is generalized into a BundleBridgeNexus that now can accept multiple inputs and a user-defined function for reducing the inputs into a single wire value that is to be broadcast, as well as a user-defined function for broadcasting that value.

• the confusingly named BundleBridgeNexus case class is renamed to BundleBridgeNexusNode
• BundleBridgeNexus is now the base class of the implementation of the reduction and broadcast wiring. It defaults to or-reducing its inputs and broadcasting identical copies of the reduction.
• BundleBridgeNexus can also be supplied a default wire value in cases where there are no input edges. This value is treated as an input by inputFn.
• aiding backwards compatibility,BundleBridgeNexus.apply produces a BundleBridgeNexusNode
• BundleBroadcast.appy is now just a factory of BundleBridgeNexus that supplies an inputFn that requires that only a single input edge be present
• object BundleBridgeNexus supplies some example input and output functions, including ones that insert timing closure registers on either input or output sides.

BundleBridgeSink is enhanced to also be able to optionally supply a gen function for producing bundles in the graph. If both source and sink supply gen functions, they must pass
chisel3.DataMirror.checkTypeEquivalence. If only sources or sinks supply a gen function, it is the one used. Neither supplying the function throws an exception. BundleBridgeNexus checks the type equivalence of all input and output edges.

Type of change: other enhancement

Impact: API modification

Development Phase: implementation