refactor cast conversion

- delegate cast type and sign resolution to TypeOf conversion
- internal support for injecting data declarations into statements
- fix size and sign of unbased unsized literals used in casts
- avoid generating many unnecessary explicit casts
- support casts which depend on localparams within procedures
- expression traversal correctly visits types within type casts
- fix typeof on expressions of net types
- handle additional edge cases for unsized integer array patterns
- preserve signedness of implicitly flattened unpacked integer arrays

test/basic/cast_struct_nested.v

@@ -0,0 +1,10 @@
+module top;
+    reg [3:0] t;
+    initial begin : blk
+        reg [4:0] x;
+        t = 1'sb1;
+        x = t;
+        $display("%b", t);
+        $display("%b", x);
+    end
+endmodule