Add new containers

[vabold]

• fixed_vector
• owning_span

Closes #177.

[malleoz]

I think you have an off-by-one error. back() should not return the same as end()

Suggested change

	T &back() {
	return *(m_data + m_size);
	}
	/// @brief Gets the last existing element in the array, or the first element if none exist.
	/// @return The last existing element in the array.
	const T &back() const {
	return *(m_data + m_size);
	}
	T &back() {
	return m_size > 0 ? *(m_data + m_size - 1) : *m_data;
	}
	/// @brief Gets the last existing element in the array, or the first element if none exist.
	/// @return The last existing element in the array.
	const T &back() const {
	return m_size > 0 ? *(m_data + m_size - 1) : *m_data;
	}