All the code-generator options can now be controlled via a json
configuration file

Extra fields can be added to the sail_state struct using a
codegen.extra_state key in the configuration json for the code
generator

If primitives want to modify the state they can be specified via
codegen.state_primops

To import such state, codegen.extra_headers can be used to add
user-defined headers to the generated .h file

This commit adds two github action to build Sail on macOS and ubuntu (both using the latest version
of each for now). These just build and don't run any tests, as we run those on our own Jenkins
server which is much faster than the github build runners.

I also fixed INSTALL.md to include brew installing pkg-config on macOS as this seems to be required.

From testing on a personal fork it seems quite email happy when it fails. Maybe that's what we want
though.

There's also a windows option but I leave that as future work...

Check in a slightly nicer stylesheet for OCamldoc generated
documentation in etc. Most just add a maximum width and increase the
font size because the default looks absolutely terrible on high-DPI
monitors.

Move val_spec_ids out of initial_check and into ast_util where it
probably belongs. Rename some functions in util.ml to better match the
OCaml stdlib.

Also, removes etc/regfp.sail and etc/regfp2.sail in favour of lib/regfp.sail