Added emplace variants of enqueue and try_enqueue

[gameraccoon]

This PR adds variants of enqueue and try_enqueue that construct queue elements in-place. This allows avoiding an extra move for potentially expensive to move objects.

The next functions were added:

• enqueue_emplace - a variant of enqueue that takes constructor arguments of an element to construct the element in-place in the queue.
• enqueue_token_emplace - a variant of enqueue that takes producer_token_t and constructor arguments to construct the element in-place in the queue (name enqueue_emplace could not be used because of collision with the previous variadic template function when using multiple arguments)
• try_enqueue_emplace - a variant of try_enqueue that takes constructor arguments (same as above)
• try_enqueue_token_emplace - a variant of try_enqueue that takes producer_token_t and constructor arguments (same as above)

Added simple unit tests to cover new functions.

The PR split into two commits: the change+tests, and refactoring to variable names.

I'm not sure whether it's a good contribution or not because it makes the code of the implementation a bit more complex, but it feels like a nice option to have for the users. Also not sure whether the chosen names fit nicely with the codebase. Feel free to make any changes.