Add methods for MMIO reads and writes

[qwandor]

This adds a bunch of different methods and macros to allow safe MMIO reads and writes of individual fields. In particular:

• get and split methods on UniqueMmioPointer<[T]> and UniqueMmioPointer<[T; _]> respectively, to go from a pointer to a slice or field to pointers to the individual elements.
• A field! macro to go from a pointer to a struct to a pointer to an individual field. (And field_shared! to do the same for SharedMmioPointer.) If [RFC] field projections v2 rust-lang/rfcs#3735 gets stabilised then we could implement that instead of needing a macro.
• OwnedMmioPointer is renamed to UniqueMmioPointer, and a new type SharedMmioPointer is added. SharedMmioPointer is to UniqueMmioPointer as &T is to &mut T. A SharedMmioPointer can be created from a UniqueMmioPointer and can be freely cloned, but fewer operations are possible on it. UniqueMmioPointer also derefs to SharedMmioPointer.
• A variety of wrapper types for fields, indicating whether they can safely be read (with or without side-effects) or written. It is important to distinguish whether reading an MMIO register may have side-effects (e.g. clearing an interrupt status, or popping from a queue) or not, because a read that has side-effects should be treated like a write and only be allowed from a unique pointer (passed via &mut) whereas a read without side-effects can safely be done via a shared pointer (passed via '&'), e.g. simultaneously from multiple threads.
• For pointers to fields with the appropriate wrapper type, safe read and write methods to perform MMIO reads and writes. On most platforms this is implemented via read_volatile and write_volatile on the underlying pointer, but on aarch64 inline assembly is used instead to work around Volatile reads and writes on aarch64 sometimes generate instructions not suitable for MMIO in protected VMs rust-lang/rust#131894. To do this safely we require that the underlying type implement the relevant traits from zerocopy.

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