Easy and comprehensive support for arrows

[topological-modular-forms]

Timothy Chow wrote:

Sometimes I want to put an arrow over something to indicate that it's a vector. If I just want a rightward-pointing arrow then \vec is usually fine; e.g., $\vec{v}$. But if I want an arrow going in the opposite direction, then the only easy solution I know is to use \overleftarrow, as in $\overleftarrow{v}$, which is clumsy. Using a harpoon instead of an arrow looks a bit more elegant to me, but seems to be complicated to implement. It would nice if extensible arrows or harpoons in both directions were easily implemented.

Darwin should have extensive support for arrows, including extensible ones. This should include also special arrows found in category theory like:

1. ${\longrightarrow}{\mkern-20mu\bullet}\mkern20mu$ for stochastic morphisms in categorical probability;
2. ⇸ for profunctors;
3. Riehl–Verity's arrows as in e.g. Definitions 1.1.23 and 1.1.25 of their book;
4. A version of ⇄ with ⊣ for adjunctions;
5. Etc.