Add some good examples from antonblanchard/vlsiffra as a regression test / benchmark

[mithro]

@antonblanchard just released the vlsiffra project. See the tweet @ https://twitter.com/antonblanchard/status/1580154261962657792 

Introducing https://github.com/antonblanchard/vlsiffra/ - a generator of fast and efficient standard cell adders, multipliers and multiply-adders. Written in Amaranth HDL, formally verified with #Yosys. #Verilator verification too With @OpenROAD_EDA + ASAP7 7nm a 32bit 3 cycle multiplier hits 2.7GHz

Some interesting aspects;

• vlsiffra is written in the Amaranth HDL language which allows it to be very configurable, including
• vlsiffra currently supports the SkyWater sky130hd, GlobalFoundries GF180MCU and ASAP7 PDKs and standard cell libraries.
• vlsiffra only requires a few standard cells (full and half adders, 2 input xor, 2 input and, inverter as well as a couple of more complicated cells (ao21, ao22, ao33)

It seems like some of the output of this tool would make excellent OpenROAD regression tests / benchmarks.

[mithro]

See associated issue on the vlsiffra report @ https://github.com/antonblanchard/vlsiffra/issues/15