A more usable alternative to std::any for C++17

Copyright (C) Giumo Clanjor (哆啦比猫/兰威举), 2020. Licensed under the MIT License.

This nonstd::any is different from std::any, in which:

• Allow constructing non-copyable non-movable types in-place via emplace<T>(args...).
• Allow constructing non-copyable types. Copying an any containing such type results in bad_any_copy exception.
• Allow constructing non-movable types. These types are constructed via an indirect pointer. Moving an any containing such type results in moving the indirection pointer.
• Moving an any will move the contained type by value with best effort, regardless the use of an indirection pointer. If not possible, the indirection pointer will be moved.
• Moving an any is always noexcept. Assumes all types are noexcept movable, even if they are not.
• Moving an any leaves the original object in empty state (i.e. always clears the moved-out object).

You may need to compile with this flag if you are using libc++ as your standard library (Android / Emscripten / etc.):

-D_LIBCPP_HAS_MERGED_TYPEINFO_NAMES_DEFAULT=0           # prevent `type_info::operator == ()` from comparing pointers.