Add enum_class option

[edsrzf]

This option allows specifying that a C++ enum should be emitted with plain enum rather than enum class. It's true that enum class should generally be preferred, but sometimes there's existing code which is already using plain enum, and porting that code to Rust becomes easier when cbindgen can emit plain enum.

This option can be overridden on a per-enum basis using a cbindgen: annotation.

It defaults to true for two reasons:

• Backward compatibility.
• enum class is probably actually what you want, in general.

[emilio]

This is ok. I wonder what will we do for conflicting stuff like enum A { B, C }; struct B { a: A }. But that's a problem that, should it matter, we already have with C.

Thanks!