Make unary - operator act as an ast_unary node. This allows for consistency (no sense...

Make unary - operator act as an ast_unary node. This allows for consistency (no sense in making unary use binstore nodes, it doesn't make much sense). It also allows for the peephole optimization on unary chains that cancel each other to take place; i.e code like "-(-a)" simplifies to "a", thus eliminating instructions.