Add option to avoid wrapping function pointer fields

[pvdrz]

Fixes #1278

[emilio]

So, if this works for someone, sure, that's fine. However I'd argue that this should be a per-field / argument / etc option.

E.g., I'd argue that we should try to handle cases like this:

typedef void (*_Nonnull FnPtr)();

void foo(FnPtr bar);

To avoid generating the Option<> only for those explicitly non-null cases.

[pvdrz]

I agree that it would be nicer to handle _Nonnull properly but at the same time it seems people don't use it that much (#1278 (comment)). I agree that it would be nicer to do this on a per-field basis but then we would need to pass a regex to this that matches both the struct and the field name.

[bors-servo]

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

[pvdrz]

Closed in favor of #2370