Allow Rust CoreAPI FFI to implement Default

[rbran]

This allow writing simpler and cleaner code when initializing FFI types, avoiding the need of core::mem::zeroed().

It's specially useful to partially initialize structs:

BNHighlightColor {
  style: BNHighlightColorStyle::StandardHighlightColor,
  color: BNHighlightStandardColor::NoHighlightColor,
  alpha,
  ..Default::default()
}

[ElykDeer]

Please add an example of this somewhere, if it makes sense in any of our existing examples/docs.

[rbran]

It's not part of the public API, because right now the Core API FFI is not exposed. It's mostly to avoid unnecessary unsafe blocks on the API implementation.