I need to get the offset of members inside sub structures.  This patch
adds sub-structure offset support for structs.

https://github.com/ruby/fiddle/commit/cf78eddbb6

* Add "offsetof" to Struct classes

I need to get the offset of a member inside a struct without allocating
the struct.  This patch adds an "offsetof" class method to structs that
are generated.

The usage is like this:

```ruby
MyStruct = struct [
  "int64_t i",
  "char c",
]

MyStruct.offsetof("i") # => 0
MyStruct.offsetof("c") # => 8
```

* Update test/fiddle/test_c_struct_builder.rb

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

https://github.com/ruby/fiddle/commit/4e3b60c5b6

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>