Implement live Faust coding with [faust~]

[bgribble]

• Add a new builtin to compile and run Faust code within an MFP DSP unit.
• Update README to give a quick intro to how to use it
• Extend the loading of custom code to all builtins
• Update wscript to capture Faust dependencies

The build on Github Actions is not exactly the same as my local -- the version of Faust is currently pinned at 2.70.3, which is about a year old. My local is 2.77.3. I had to do some ugly stuff in builtin_faust.c to get it to compile with the older version of Faust, which I believe has some bugs in its C headers.