docs(cordyceps): use ptr::addr_of_mut! instead of casts

[hawkw]

Currently, the cordyceps tests and docs show the use of a #[repr(C)]
struct with the Links field as the first member of the struct, and use
a cast from NonNull<Self> to NonNull<Links<Self>> in order to
implement the Linked::links method. This is done to avoid the creation
of a temporary reference by dereferencing the pointer and borrowing the
Links field, which Stacked Borrows dislikes.

This approach is not ideal for a couple of reasons:

• It performs a layout-dependent cast, and therefore requires the
  #[repr(C)] attribute be present to ensure soundness. This means
  that forgetting to add that attribute may result in miscompiles.
  Also, relying on struct layout makes me feel uncomfortable and we
  should try to avoid it where possible.
• It means that a given struct may not contain multiple types of
  Links for different intrusive data structures. This is because the
  layout-dependent cast requires the Links field to be the first
  member of the struct, and multiple fields obviously cannot be the
  first. :)

We can also avoid the creation of a temporary reference by using the
core::ptr::addr_of_mut! macro to offset a pointer to a subfield of the
pointed type. This avoids the disadvantages of the cast while still
passing Miri.

Therefore, this branch updates the cordyceps examples, docs, and
tests, to show the use of addr_of_mut! rather than casting. In
addition, I've added a section to the Linked trait documentation
explaining why it's necessary to avoid temporary references, and
discussing why addr_of_mut! should be used for this purpose.

In the future, we may want to consider adding a macro (or a derive
attribute?) that generates correct Linked implementations using
addr_of_mut!, to make it easier to use intrusive structures safely...