GitHub Actions CI

[sjkelly]

In #33 I discovered that yosys is in the Debian/Ubuntu repos, which should make running synthesis on GitHub actions much easier. Nextpnr and symbiyosys are not in the repos, so we can't run a full flow on github actions. However only running synthesis should be okay for catching most bugs.

To actually make this happen the makefile needs to be adjusted so only synthesis can be run. Currently we generate PLLs before synthesis but without icestorm or prjtrellis we can't do this. There are a few options:

• Pre-generate PLL modules/parameters and keep them in the repo
• Pass a macro define into synth to ignore PLL instantiation similar to what is done with make formal

[sjkelly]

33 works by pulling out auto generated PLL modules (buggy atm) and using abc2 rather than abc9 for techmap.

[sjkelly]

Closed in #37. We now have a full .v->.bit and formal verification running on CI.