Add flag to ignore type blocklist when computing derive information

[jethrogb]

Fixes #1454

I also prototyped adding the following method to ParseCallbacks but I think this PR is nicer:

/// This will be called to determine whether a particular blocklisted type
/// implements a trait or not. This will be used to implement traits on
/// other types containing the blocklisted type.
///
/// * `None`: use the default behavior
/// * `Some(true)`: `_name` implements `_derive_trait`
/// * `Some(false)`: `_name` doesn't implement `_derive_trait`
fn blocklisted_type_implements_trait(
    &self,
    _name: &str,
    _derive_trait: DeriveTrait
) -> Option<bool>;

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[jethrogb]

This PR may not be sufficient, will provide additional tests later.

[jethrogb]

So the main problem is that when using external crates to provide Rust definitions as in #1454 (comment), bindgen can't know what traits are/aren't implemented by those crates. I don't think there's a way around implementing blocklisted_type_implements_trait for a comprehensive solution.

[emilio]

Looks good, thanks!

[jethrogb]

Uh sorry but this was not meant to be merged anymore, see my last comment.

[emilio]

Ah, I assumed that given the other patches and this was open, it was intended to be merged after all. I'll revert, thanks.