Gets this out of the global namespace.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

- Fixed a bug where pushing items over queue's size left it in a corrupted state
- For non-trivial types, have clear() and pop() run destructors
- Added emplace(args...)
- Added empty()

FixedSizeQueue has semantics of a circular buffer,
so pushing items continuously is expected to keep overwriting oldest elements gracefully.

Tests have been updated to verify correctness of a previously bugged behaviour
and to verify correctness of destructing non-trivial types