Request: enum_type Additions

[drewr95]

I'd like a couple of possible additions/improvements using enum_type.h:

1. I'm getting a linter warning when using enum_type because one of the constructors is missing an explicit marker.
   warning: single-argument constructors must be marked explicit to avoid unintentional implicit conversions [google-explicit-constructor,hicpp-explicit-conversions]
2. It would be nice to utilize etl::to_underlying for the underlying value_type typedef so that duplication is removed. If the intent is to not use underlying types for enum, maybe it's best to have a C++11+ version vs a legacy version?