Milestone SableCC 4.0

This is a complete rewrite of SableCC to bring many new features:

• Improved lexer engine (additional operators, lookahead, and more).
• Improved parser engine (linear approximate LR(K) parsing, semantic selectors, and more).
• Improved conflict reporting (enabling LR grammar debugging).
• Flexible code generation (to enable back-ends for various languages).
• Improved syntax for CST->AST transformations.