Major redesign of expr width/sign detecion (verilog/ast frontend)

tests/simple/signedexpr.v

@@ -0,0 +1,18 @@
+module test01(a, b, xu, xs, yu, ys, zu, zs);
+
+input signed [1:0] a;
+input signed [2:0] b;
+output [3:0] xu, xs;
+output [3:0] yu, ys;
+output zu, zs;
+
+assign xu = (a + b) + 3'd0;
+assign xs = (a + b) + 3'sd0;
+
+assign yu = {a + b} + 3'd0;
+assign ys = {a + b} + 3'sd0;
+
+assign zu = a + b != 3'd0;
+assign zs = a + b != 3'sd0;
+
+endmodule