Fix duplicate definition of stx::in_place and related

[jwnimmer-tri]

Closes #4.

In some cases, optional.hpp and variant.hpp still could not be included
in the same translation unit:

(1) If the user included variant.hpp first and optional.hpp second and
if <variant> was available, we'd see duplicate inplace_t and inplace.

(2) If the user included optional.hpp first and variant.hpp second and
if <optional> was available, we'd see duplicate inplace_t and inplace.

(3) If the user included optional.hpp first and variant.hpp second and
if <optional> was not available, we'd see duplicate in_place_type_t,
in_place_type, in_place_index_t, and in_place_index.

This improves on 6da04dc.

This is similar to the fix proposed in #8, but goes a step further and removes the in_place_type_t and in_place_index_t from optional.hpp -- they are not related to using optional (only variant and any), and when we're using <optional> or <experimental/optional> they were not being aliased anyway, so they were only conditionally provided by stx. It seemed better to remove them, instead of trying to make them play nice with variant.hpp.

[jwnimmer-tri]

I'm no longer using stx (I'm C++17-only now, yay!), so I'll close this out.