* 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>