Add option to avoid wrapping function pointer fields or arguments

[pvdrz]

This PR adds a new flag --non-null-fn-ptr=<PATH> that can be used to avoid wrapping function pointer arguments or fields in Option if their path matches the <PATH> regex.

For example if we had the following input headers:

typedef struct foo {
    int (*bar)();
} foo;

typedef struct baz {
    int (*bar)();
} foo;

and we were to call bindgen without this flag we would get an output like this:

pub struct foo {
    pub bar: Option<unsafe extern "C" fn() -> c_int>,
}

pub struct baz {
    pub bar: Option<unsafe extern "C" fn() -> c_int>,
}

However, if we call bindgen --non-null-fn-ptr="foo::.*" the output would be:

pub struct foo {
    pub bar: unsafe extern "C" fn() -> c_int,
}

pub struct baz {
    pub bar: Option<unsafe extern "C" fn() -> c_int>,
}

I understand that the ideal would be to change bindgen so it could process nullability attributes instead but it seems those attributes are not used very often and then we would require people to change their c code to achieve the same.

[bors-servo]

☔ The latest upstream changes (presumably ed2d06e) made this pull request unmergeable. Please resolve the merge conflicts.

[bors-servo]

☔ The latest upstream changes (presumably 2034a0f) made this pull request unmergeable. Please resolve the merge conflicts.

[Bromeon]

What happened to this PR?
No feedback in half a year?