Add vpr from verilog-to-routing (for Xilinx Series 7 support)

[Tim 'mithro' Ansell]

VPR can be used to target Artix 7 and other Xilinx 7 series parts as well as the QuickLogic EOS S3 parts.

• https://github.com/SymbiFlow/vtr-verilog-to-routing
• https://github.com/verilog-to-routing/vtr-verilog-to-routing
• https://www.verilog-to-routing.org

[Ed Bordin]

Thanks for the suggestions Tim. I think it would be great to expand this package to support targets other than ICE40 and ECP5. My main considerations at the moment are (these apply to #52 #53 too):

1. Package size: currently the whole thing is about 100MB compressed. If adding new tools significantly increases that then I would probably have to think about breaking things up into separate packages which is a can of worms I have not yet had to open.
2. Portability: Everything is statically linked in this package which makes distribution a breeze. But any tools without static build targets become a lot more work as I have to individually patch the build for each target to make it work and possibly contribute this upstream. I have also been trying to maintain feature parity between all platforms which often requires extra work to get tools building on platforms other than linux. I have been wondering if a docker-based environment would be a good complement to this approach as it would be much easier to just run make && make install on a bunch of tools without worrying about static linking, and it would also make things like verilator and cxxrtl easier to set up, but see next point:
3. My time: I have spent a lot of weekends slowly getting this to where it is and it is at the point where I was nearly ready to stop adding new features and just maintain it. I have a bunch of dev boards sitting there waiting to be played with...