Workaround for compilation failure on MSVC

[ednolan]

This commit depends on
#27.

There is implementation divergence as to whether the following code is
accepted:

template <typename T>
struct foo {
    static constexpr auto baz() {
        return 0;
    }
    using quux = decltype(baz);
};

It is accepted by GCC and Clang, and rejected by MSVC and sometimes
by EDG: https://godbolt.org/z/dW964rqec

Because the calculation of the iterator_category depended on code of
this form, previously, MSVC would fail to compile iterator_interface
with this error:

error C3539: a template-argument cannot be a type that contains 'auto'

In order to work around this issue, the static member function-based
metaprogramming has been replaced with the use of std::conditional_t.

After this change, MSVC successfully compiles and runs all the tests.