c2rust-refactor: Implement deep structural comparison of ADTs by comparing all variants

[ahomescu]

Fixes one set of issues with curl (edit: and nginx) caused by a fake equivalence between core::ffi::c_void and libc::timezone:

error[E0308]: mismatched types
    --> src/src/tool_util.rs:15:70
     |
15   |     ::libc::gettimeofday(&mut now as *mut _ as *mut ::libc::timeval, crate::__stddef_null_h::NULL);
     |     -------------------- arguments to this function are incorrect    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `*mut timezone`, found `*mut c_void`
     |
     = note: expected raw pointer `*mut timezone`
                found raw pointer `*mut c_void`
note: function defined here
    --> /home/ahomescu/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libc-0.2.177/src/unix/linux_like/linux/gnu/mod.rs:1126:12
     |
1126 |     pub fn gettimeofday(tp: *mut crate::timeval, tz: *mut crate::timezone) -> c_int;
     |            ^^^^^^^^^^^^
Som

[ahomescu]

This fixes #266, we should close that bug when this lands.

[kkysen]

.all_fields() does a flat map over .variants() and then .fields(), which is what we're doing here. Is the issue that the flattened iterator may have the same elements, but the boundaries of each variant are different?

[ahomescu]

That's right, plus that it doesn't account correctly for empty variants. For example, it considers the following two enums equivalent:

enum Foo {
  One,
}
enum Bar {
  One,
  Two,
}