Usage of references is in conflict with use for MMIO

[RalfJung]

As described at rust-embedded/volatile-register#10, there is a big problem with having things like &VolatileCell and using that for volatile accesses: these references are marked as dereferencable and the compiler is allowed ti insert spurious (and non-volatile) read accesses.

There currently is no way in Rust to have references without permitting the compiler to do spurious reads.

Some more discussions for context:

• UCG discussion
• A libstd issue caused by this behavior
• Several threads on IRLO