Introduce patch

[guillep]

Right not the application of changes (and thus also reverts) is a bit of a mess:

• part of it depends on monticello loading
• part of it is in visitors

Monticello loading is nice because it sorts the changes and applies them in the right order.
However, by default monticello does that in a per-package basis.

We need to do it in a per-repository basis. Otherwise inter-dependent changes from different packages in the same repository may be loaded in the wrong order and raise uncomfortable load warnings.

This could be solved by introducing a repository patch abstraction on iceberg. Visitors should then only build a patch and the patch be installed in a single-shot. Maybe monticello's patch/loader could be reused?