Added module for sign extension. Needs test cases

modules/sign_extender.sv

@@ -0,0 +1,13 @@
+`ifndef SIGN_EXTENDER_H
+`define SIGN_EXTENDER_H
+
+module sign_extender #(in_width=4, out_width=5)(
+    input logic [0:0] op,
+    input logic [2**in_width-1:0] val,
+    output logic [2**out_width-1:0] ext_val
+);
+    always_comb begin
+        assign ext_val = (op) ? {{(2**(out_width-in_width)){val[2**in_width-1]}}, val} : ({{(2**(out_width-in_width)){1'b0}}, val});
+    end
+endmodule
+`endif