Register: Add a truly type-safe data structure for registers.
Add tests.
Register: Add simple non-typesafe "enum class".

The static constexpr members require an external definition for the linker.
This is a known C++11 problem and makes static constexpr pretty useless.

enum class cannot have cast operators.
Apparently a "design decision" by the committee.
Register: Using combined enum class and struct for type.
This however does not fully overcome the enum class casting problem.
Register: Settle on non-typesafe struct {enum}.
Register: Remove unittest, there isn’t really anything to test here.
Register: xpcc::BitX moved to bit constants.