Implement new MLscript syntax

[LPTK]

This is a tracking issue for the current effort to reimplement the parser, and with it change and extend the syntax of MLscript to make it more practical and expressive.

Among other things, the new syntax will allow nested classes, and class definitions will be allowed to take value parameters. The unnesting of classes for efficient compilation is briefly discussed here: 82d89e0#commitcomment-78640652.