Enable arbitrary register address and value sizes

[sfe-SparkFro]

Most devices have 8-bit register addresses with 8-bit values. However some devices have 16-bit register addresses, or 16-bit register values, or even 32-bit (eg. the XM125 uses 16-bit addresses with 32-bit values, see section 3 here).

Rather than making special functions for every possible permutation, I think it would make sense to have a single set of read/write functions that can accept any address/value size. The user would need to initialize the size and endianness of the addresses and values once at the start.