Implement synchronous procs

Synchronous procs is a proposal which implements procs as [synchronous data flow](https://en.wikipedia.org/wiki/Synchronous_Data_Flow) . Proposal: https://google.github.io/xls/design_docs/synchronous_procs/

The primary advantage is that flow control and FIFOs can be eliminated on internal channels within the hiearchy. The disadvantage is that this only works undercertain conditions (outlined in the design doc).