FFI compatible token interface

[jackcmay]

Token account definitions were not FFI compatible because of the use of Option<T>

• use an explicit boolean instead
• autogenerate C headers using cbindgen

[jackcmay]

@garious @t-nelson fyi, using cbindgen to create C headers

@garious Option<T> is not FFI friendly so I replaced with an explicit boolean. Another option is to define our own "Option" that uses #[repr(C)], thoughts?

[garious]

Our own Option sounds better to me. It's a fundamental building block these days.

Why add the auto-generated code to git?

[jackcmay]

@garious I'll give our own options a shot, probably want that to live in our SDK eventually.

Checking in the header has two advantages, non-rust projects that want to pull use the header don't have to use Rust to generate it. Also its more explicit when someone intentionally or unintentionally makes a change that affects the public interface.

[jackcmay]

@garious I added a repr(C) version of Option, was this the route you had in mind?

[jackcmay]

Will move option.rs to the SDK once/if approved

[jackcmay]

@garious What do you think of our use of own OptionReprC and putting it into the SDK?

[garious]

@jackcmay, no objection. Buy maybe name it COption? Analogous to ffi's CString?