Create read-only/write-only/read-write variants of VolatileCell about tock HOT 5 CLOSED

Here's a sketch of how that might look:
https://play.rust-lang.org/?gist=c72fff775926f381bf6371c2de24e995&version=stable&backtrace=0

from tock.

Would also be useful to have a handful of common, more specific types for as type parameters to VolatileCell for registers. For example, it would be nice to have types that:

• Take up 32 bits in memory but only have 16/8 bits meaningful
• Expose a bit setting/clearing interface
• Others?

from tock.

Bit set, clear, and toggle (xor) are the standard triple for register interfaces

from tock.

Agreed. Plus it's fairly common to have a register that controls both set & clear, so you end up doing something like:

registers.my_reg.set(registers.my_reg.get() & !(1 << BITNAME));

from tock.

Essentially solved by #658

from tock.