Shankara/picus backend

[shankarapailoor]

Preliminary version of the Picus backend for Ziren

• Includes an implementation of AirBuilder called PicusBuilder to build Picus programs
• Added a data structure called PicusInfo which tracks the inputs, outpts and selectors for a Chip.
• Extended the chip interface to include a picus_info function which produces a PicusInfo structure for that Chip. Currently this function is implemented manually but we can create a macro to generate this automatically.