Replace Javascript with a simple expression evaluator for GC validity check

[pont-us]

Javascript is going to be excised from Java soon. If I have to bundle something to evaluate the GC expression anyway, there are far less bulky options. https://stackoverflow.com/questions/3422673/ seems like a good start.

[2020-11-21 Sat] Nashorn is gone as of JDK 15. See https://stackoverflow.com/questions/62259515/ for some alternatives; apparently I could use GraalVM's JS engine for Javascript without graalifying the whole of PuffinPlot. Also Rhino's still alive.

Dropping Javascript entirely and using an expression evaluator (and keeping Jython) seems sensible. I've never heard of anyone actually using Javascript in PuffinPlot.

Possible candidates:

• https://dzone.com/articles/even-faster-java-expression
• https://github.com/darius/expr
• https://github.com/scireum/parsii
• https://github.com/nathanfunk/jep-java-gpl