etl/type_traits.h: Add support for type_identity

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[jwellbelove]

Did you add a unit test?

[tigran2008]

Sorry, I overlooked that part :/

I'm away from my computer right now, but would the following be enough if I added it later?

Given this function:

template <typename T>
T add(T first, typename etl::type_identity<T>::type second) {
    return first + second;
}

The result of add(2.5, 1) would be checked if it was equal to 3.5

[jwellbelove]

That looks OK.

[tigran2008]

Did I do this right? 😅

[tigran2008]

Well, I changed the CHECK into a CHECK_CLOSE.