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.sv

@@ -0,0 +1,14 @@
+module top;
+    typedef struct packed {
+        logic x, y;
+    } S;
+    typedef struct packed {
+        S x, y;
+    } T;
+    T t;
+    initial begin
+        t = 1'sb1;
+        $display("%b", t);
+        $display("%b", 5'(t));
+    end
+endmodule