ast: Add support for $sformatf system function

Signed-off-by: David Shah <[email protected]>

tests/various/sformatf.ys

@@ -0,0 +1,12 @@
+read_verilog <<EOT
+
+module top;
+	localparam a = $sformatf("0x%x", 8'h5A);
+	localparam b = $sformatf("%d", 4'b011);
+	generate
+		if (a != "0x5a") $error("a incorrect!");
+		if (b != "3") $error("b incorrect!");
+	endgenerate
+endmodule
+
+EOT