Add `offsetof` so we can get the offset of members

I want to get the offset of fields inside structs, but I don't want to
instantiate the struct.  I need to embed the offsets inside machine
code, and I can't get the offsets without calling `new` on the struct.

This commit adds an `offset` method so you can get the offset of a
member without instantiating anything.  You can do:

```ruby
C.rb_control_frame_t.offsetof(:sp) #=> 8
```

I don't think this implementation is perfect, you can only get immediate
fields.  But it is better than nothing!

lib/ruby_vm/mjit/c_pointer.rb

@@ -54,6 +54,12 @@ module RubyVM::MJIT # :nodoc: all
           # Return the size of this type
           define_singleton_method(:sizeof) { sizeof }
 
+          # Get the offset of a member named +name+
+          define_singleton_method(:offsetof) { |name|
+            _, offset = members.fetch(name)
+            offset / 8
+          }
+
           define_method(:initialize) do |addr = nil|
             if addr.nil? # TODO: get rid of this feature later
               addr = Fiddle.malloc(sizeof)