Finalize RegisterPack implementation

[sendyne-nicocvn]

(This issue is a continuation of the discussions and suggestions in #7)

Before merging the register pack implementation the following items need to be decided:

• [x] Implement an enumeration type for register width (@hak8or) to limit the possibility of typo/error and enforce supported widths.
• [x] RegisterPack takes as template argument the size of the pack in bytes; we keep it this way or we decide that the size should be in bits.
• [x] PackedRegister takes as template argument the offset with respect to the pack base in bits; we keep it this way or we decide that the offset should be in bytes.
• [x] Naming: should we change RegisterPack and PackedRegister to something else (Peripheral and PeripheralRegister, Device and DeviceRegister) ...
• [x] Alignment checking is currently done in a "naive" way; this should be revised using a safer implementation and the limitations should be documented (in a pack, of register of size N bits can only be defined is the pack base is aligned on a N bits boundary and if the register address is at an offset multiple of N),
• [x] Update documentation accordingly.

[sendyne-nicocvn]

In 8cdc89336f85e254a8a6a094bf42030c05bbda29 the following is now available:

• an enum type (RegBitSize) is now used to represent register sizes,
• 64-bit registers are now supported,
• in PackedRegister alignment is properly checked through static_assert.

[sendyne-nicocvn]

@hak8or can you take a look at the quick start and API documentation; just want to be sure it is clear and without (too much) mistakes/typos.

The only items left are those related to naming and how we express sizes ... any input is welcome.

[sendyne-nicocvn]

So the documentation have been updated and is now consistent with the current implementation. Since all seems fine with the interface (bits vs bytes and naming) I am closing this issue and will merge to master.