Add microcontroller reserved register abstraction

[apcountryman]

Add microcontroller reserved register abstraction (::picolibrary::Reserved_Register).

• [x] The Reserved_Register class should be defined in the include/picolibrary/register.h/source/picolibrary/register.cc header/source file pair
• [x] The Reserved_Register class should have the following template arguments:
  • [x] typename T: The register's underlying integral type
• [x] The Reserved_Register class should have the following member types:
  • [x] using Type = T;: The register's underlying integral type
• [x] Documentation
  • [x] docs/register.md