`AlgPart` constructors require `new`, `____SiGNMove` helpers cannot be called with `new`

[lgarron]

Example:

new Sequence([BareSiGNMove("R")])

It's probably confusing to beginners to know when to use new. TypeScript and ES6 require new for classes, and I don't want to create a helper for every class. So the main workaround I see here is to make SiGNMove an abstract class, and SiGNMove helpers into actual subclasses of SiGNMove.