Bitwise operator templates for enums

When an enum is used to define bitwise flags that are meant to be
combined (FLAG_ONE | FLAG_TWO), the resultant value needs to be cast
back to the enum type in order to be stored in a variable of the same
type as the enum (as the result is a value of underlying_type of the
enum).

This adds a macro that allows templated bitwise operators to be defined
so that the enumerators can be used more cleanly in code, while
preserving type-safety.

Test:  unit-tests
Change-Id: I6f945236e430b644758d2030c9c358c655d84941