CI wishlist

[martijnbastiaan]

@alex-mckenna wanted to work on the CI, so here's my wishlist.

Make it fast:

• Improve compile times (this is the most invasive one, so we might want to do it last?)
• Parallelize (we probably need needs?)!
  • Split out VHDL/Verilog/SystemVerilog tests
  • Haddock
  • doctests
• Lazy load Haskell blackboxes (edit: actually I don't think this is relevant. lazy-loading primitives is already there as long as you don't use json files, but inline primitive annotations)
• Clash remains the slowest thing in our tests, even for very small tests: look into Clash startup performance.
• add Verilator to the testsuite (Generate Verilator-friendly verilog #418)

Other things:

• Setup cachix
• Test with GHC 8.10
• Update the docker image to Ubuntu 18.04
• Performance tracking
• Setup master.hoogle.clash-lang.org