Register sizes are now represented by an enumeration type. This limits the
possibility to typeset unsupported register sizes, and make the interface easier
to read.
See #12.
* For trivial read/write operations (over whole register) simpler implementations are now used to avoid unnecessary load/store instructions.
* Merge write implementation has been revised to provide better performance when manipulating constant values.
* `is_set` and `is_clear` are now available for all Field-based types.
This closes#8.
