Update bindgen to 0.71

[jbourassa]

Bump bindgen to 0.71 in hope to fix compilation on Windows with recent clang. IIUC, latest bindgen should not have that problem.

I ran into 2 issues when bumping the crate:

1. ABI checks that referenced either private fields or out-of-scope structs:

   const _: () = {
       ["Size of RString"][::std::mem::size_of::<RString>() - 40usize];
       ["Alignment of RString"][::std::mem::align_of::<RString>() - 8usize];
       ["Offset of field: RString::basic"][::std::mem::offset_of!(RString, basic) - 0usize];
       ["Offset of field: RString::len"][::std::mem::offset_of!(RString, len) - 16usize];
       ["Offset of field: RString::as_"][::std::mem::offset_of!(RString, as_) - 24usize];
   };

   I fixed it by patching those blocks with a use crate::internal::*; and getting rid of the rb_sys__Opaque__ prefix. Not sure what underlying change made this a problem though.

2. Missing proc_macro2::Literal::c_string when testing against magnus. Hence the proc_macro2 bump.

With those changes, magnus tests pass on my machine. Let's see what CI says.

[jbourassa]

Not familiar with bindgen nor syn -- hopefully this makes sense. I think it preserves the intent, that is, enusre the types are ABI-compatible.