Do not inherit from Struct

Just better practice to not inherit from Struct. Not much changes but the ancestor chain is cleaner.

```ruby
class A < Struct.new(:x); end
B = Struct.new(:x)

A.ancestors
# => [A, #<Class:0x007f9ba12cd168>, Struct, Enumerable, Object, Kernel, BasicObject]
B.ancestors
# => [B, Struct, Enumerable, Object, Kernel, BasicObject]
```