himbaechel: Adding a xilinx uarch for xc7 with prjxray

[gatecat]

This PR ports the xray/xc7 side (i.e. everything without rapidwright/ultrascale) of the old nextpnr-xilinx repo into himbächel.

This is not finished or working, and it's a long way from being anything production-grade, but it's a useful test of the himbächel framework for something more complex.

As well as it just not quite producing working bitstreams yet blinky at least works now, there are several other key issues to be investigated and solved:

• no timing data
• router isn't tuned (delay estimates etc) so performs terribly
• router2 doesn't work at all because of some problem relating to how the new constants API is implemented (this arch is also the first serious test of that)
• some IOLOGIC code is missing
• the database build performance and memory footprint needs improving for bigger devices, maybe moving away from python eventually
• it'd be nice to use the "new" support for cell/bel pin mappings and bel buckets instead of the approach nextpnr-xilinx took at the time of just changing cell type and port names as needed

[hansfbaier]

Would be great so see how well this performs with one of the larger parts, like the Kintex 420/480T which I have contributed to prjxray.