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

[ahomescu]

Fixes one set of issues with curl 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