Translate C++ enums as Rust integers

[lucic71]

The previous translation, C++ enums -> Rust enums had two weak points:

• it did not allow enums that have equal variants, for example enum E { A = 1, B = A }
• operations on the enum could not be used in const context, for example: int global = E::A != E::B. != for enums is not const so it cannot be used in const expressions like initializing a global variable

The new translation does C++ enums -> Rust integers and the name of the values becomes E_A instead of E::A.