Use type system to indicate that peripheral instance pointers are not null

[apcountryman]

Use type system to indicate that peripheral instance pointers are not null.

• [x] Change return type to Not_Null<Type *> https://github.com/apcountryman/microlibrary/blob/459f2f9009503a99280abe10c0a7f74b91e3202a/libraries/microlibrary/ANY/ANY/include/microlibrary/peripheral.h#L57