(WIP) Simplify management of sources

[copernico]

Currently, we rely on git commits as the "unit of storage" which has the advantage that we can reuse git signing mechanism to check the authenticity and integrity of statements; this is however inefficient when looping over may statements (not viable with a few hundreds).

This PR drops this mechanism in favour of plain filesystem. Statement files are iterated on the filesystem, the signature is stored as a separate file.

Additional consequence: we may avoid requiring that statements be in a fixed directory under the root of a repository (which in turn forced us to have a separate branch just for statements)