Supporting using `readonly` attribute on `struct`

[Chaoses-Ib]

#151 added support for using readonly attribute on fields. But when using it on a struct it'll be ignored and emit a "unused variable" warning. Is it possible to support using readonly attribute on struct too? This can make the code clearer to read if there are many readonly fields.

[Spxg]

You mean to use readonly on the struct, and then only generate getters for all the public fields, right? That sounds reasonable.