Enum Class Bitmask Operators

A scoped enum gives you type safety but loses the bitwise operators. Add them back for a peripheral flags enum without giving up the type checking.

Constraints

• Enum class `Flags : uint8_t` with Rx, Tx, Error, Overrun.
• Provide operator| , operator& , operator~ and a has() helper.
• Mixing Flags with a raw int must not compile.
• Operators must be constexpr.
• No macros.