Mitigate invalid transmute when checking memory initialization

[artemagvanian]

This PR addresses another aspect of #3324, where delayed UB could be caused by transmuting a mutable pointer into the one of incompatible padding. It also adds a check to error whenever transmuting between two types of incompatible padding:

// 5 bytes of data + 3 bytes of padding.
#[repr(C)]
#[derive(kani::Arbitrary)]
struct S(u32, u8);

/// Checks that Kani catches an attempt to access padding of a struct using transmute.
#[kani::proof]
fn check_uninit_padding() {
    let s = kani::any();
    access_padding(s);
}

fn access_padding(s: S) {
    let _padding: u64 = unsafe { std::mem::transmute(s) }; // ~ERROR: padding bytes are uninitialized, so reading them is UB.
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[celinval]

Just one minor comment. Thanks!

[celinval]

It looks good. Just please add the new invariant to distinguish the unsupported vs the trivially false cases. Thanks!