Use bindgen to generate API code

[wks]

• Somewhere in mmtk-ruby we compute struct offsets using the memoffset crate. That carries a dependency on the nightly version of Rust. Remove it.
• Use rust-bindgen to generate Rust counterparts of Ruby structs in C
• Use cbindgen to generate the mmtk.h used in the ruby repo.

[wks]

We have been using cbindgen to generate the mmtk.h header since this commit: 86ff3d2

We will need to look directly into Ruby objects when we start to optimise object scanning. Then we'll use rust-bindgen to generate the binding in the other direction.