Add multiplication and division compound assignments

[lam2mo]

less-java/src/main/antlr/LJ.g4

Lines 144 to 145 in 462beed

	ADDASGN: '+=';
	SUBASGN: '-=';

[lam2mo]

Maybe remove ASGN as well? Causes too many if (n == 0) bugs.

[MrBartWolf]

Files that I know need to be changed for this include: LJ.g4, ASTBinaryExpr.java, LJStaticAnalysis.java, and LJASTInferTypes.java.