Add the walking reflexive pair

[ScriptRaccoon]

This PR adds the walking reflexive pair, the category generated by a morphism $i : 0 \to 1$ and two morphisms $p,q : 1 \rightrightarrows 0$ with $pi = qi = \mathrm{id}_0$.

This PR resolves #122.

TO DECIDE

• has cofiltered-limit-stable epimorphisms
• has cosifted limits
• has filtered-colimit-stable monomorphisms
• is sifted

[ScriptRaccoon]

@varkor What would be the procedure to add that nice diagram with quiver as a vector graphic? LaTeX code:

\begin{tikzcd}[column sep=40pt,nodes={inner sep=3pt}]
0 \ar{r}[description]{i} & 1 \ar[bend left=40]{l}{p} \ar[bend right=40]{l}[swap]{q}
\ar[loop right, distance=3em, start anchor={[yshift=0.5ex]east}, end anchor={[yshift=1ex]east}, in = 60, out=0]{}[right]{ip}
\ar[loop right, distance=3em, start anchor={[yshift=-0.5ex]east}, end anchor={[yshift=-1ex]east}, in = -60, out=0]{}[right]{iq}
\end{tikzcd}