static cast: support changing size and signedness

Support SystemVerilog Static Cast
- size
- signedness
- (type is not supposted yet)

Fix #535

frontends/ast/ast.h

@@ -76,6 +76,7 @@ namespace AST
 		AST_TO_SIGNED,
 		AST_TO_UNSIGNED,
 		AST_SELFSZ,
+		AST_CAST_SIZE,
 		AST_CONCAT,
 		AST_REPLICATE,
 		AST_BIT_NOT,