Pipeline multiply and other primitives

[mikeurbach]

This came up over here: #909 (comment). The multiplier and some of the other non-trivial primitives use multiple stages internally, and it should be possible to pipeline them with II=1. Now that we are starting to push on statically timed pipelines this could yield some nice improvements.

[rachitnigam]

Outlining an implementation sketch for anyone interested in implementing this. The current [std_mult_pipe][smp] is actually already pipelined internally so that Xilinx tools infer them as DSPs. However, the interface does not expose any way to use the multiplier in a pipelined way because it only implements a one-sided ready-valid interface. To make this work, we can start off with the primitive and expose a proper, double-sided read-valid interface. This new primitive should probably be exposed in a separate library file. Next, we can change the code generation for SCF programs to explicitly model reads and writes to/from the multiplier when generating the pipeline so that they can be pushed into the right stages.

[rachitnigam]

One idea for doing this is using the primitives generated from Filament designs and exposing two ports: One that allow access to the pipelined execution and another one that registers the output. If a design doesn't use the latter, it'll get optimized into a purely pipelined operator

[rachitnigam]

Once we wrap up #1725, we can start working on this. The new interface is flexible enough to allow for modules to be pipelined.