Flaw in offset_of and span_of causes SIGILL, drops uninitialized memory of arbitrary type on panic in client code

Affected versions of this crate caused traps and/or memory unsafety by zero-initializing references.
They also could lead to uninitialized memory being dropped if the field for which the offset is requested was behind a deref coercion, and that deref coercion caused a panic.

The flaw was corrected by using MaybeUninit.