Better register and bitfield support

[bgamari]

Currently the register macros do their job but not terribly well. Read/modify/write is an extremely common task, especially when modifying a single bitfield within a register, yet this task is rather painful with the current interfaces.

The mchck project has shown that having bitfields represented in the type system makes code both substantially easier to read and write.

There have been discussions about adding native bitfield support to Rust in the past. A Rust issue has been created, as well as an RFC. Servo has a bitfield macro that may be ripe for plucking and improvement. Unfortunately, the macro system isn't terribly great at generating accessors until concat_idents! is useful.

[bharrisau]

Suggest we track this in #3. Bit banding support is important too and using a bitfield macro won't support the bit-banding properly.

[farcaller]

A quick note on this: I've been thinking of masked_set_XXX to do the register access, but that will bloat the code at reg_rw definitions.

[bgamari]

@bharrisau, thanks! Somehow I missed that one.