cursing-around package

A package for multiple cursor management. Loosely inspired by multi-cursor, the main differences feature-wise being :

• symmetry between the use of buffer and screen coordinates
• symmetry between the four directions (hence the around)

The status is very embryonic. Before the package being usable, I need at least cursing-around-screen:move-cursors-up to be functionally equivalent to core:move-up.