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!
I noticed this while running test_yjit with --mjit-call-threshold=1,
which redefines `Integer#<`. When Ruby is monkey-patched,
MJIT itself could be broken.
Similarly, Ruby scripts could break MJIT in many different ways. I
prepared the same set of hooks as YJIT so that we could possibly
override it and disable it on those moments. Every constant under
RubyVM::MJIT is private and thus it's an unsupported behavior though.
