Add support for sub-indices

[hannobraun]

The DW1000 register interface allows for addressing within a register by using a sub-index. This can be required for accessing larger registers, especially since the nRF52832's[1] SPIM peripheral is somewhat restricted in the buffer sizes it supports (also see jamesmunns/nrf52-hal#14).

[1] The nRF52832 is especially relevant to this crate, as it's the microcontroller used on the DWM1001 board.

[hannobraun]

There's one open question when it comes to implementing support for sub-indices: When accessing only a part of a register, what happens to the getter/setter methods that are generated as part of the R/W types for each register?

Here's a wild idea I just had:

• Add two type parameters to the generated R/W types that indicate which part of the register is being accessed. They default to 0 and the length of the register (in bytes), but can be set to define any subset of the register.
• Each getter/setter is only implemented, if the range defined by the two type parameters includes all the bytes that contain the field (using a where clause on the impl block).

I think right now this is mostly science fiction, but it should be possible once we get const generics. Maybe it's even possible to get something like this using typenum.

[hannobraun]

I'm closing this issue. Support for sub-registers has been added in #22. A more generic mechanism, as described here, seems unnecessary, as most registers are only a few bytes long, and I don't think the additional complexity is worth it to save a few bytes in an SPI transaction.

The only exceptions that I can think of are RX_BUFFER/TX_BUFFER, and there's a dedicated issue for those.