Change license from CeCILL v2 to MPL-2.0 (more permissive)

[DmxLarchey]

Following the issue #207, I propose this relicensing PR.

No functional change at all here since only at most one line of comment is updated for each .v file.

However, please check that the two added files MPL-2.0_LICENSE.txt and scripts/relicense_to_MPL.txt are properly placed.

Possibly we could consider a licenses subdir? Also about the new scripts subdir:

• is it properly placed? named?
• it is here only to remind what was done.

The CeCILL_LICENSE.txt file is kept for the moment but could be pruned easily in a later update.

[Zimmi48]

Usually, when a project is not dual-licensed, it is recommended to keep a single license file and to name it LICENSE. This helps both humans and automated tools understand more quickly what the project license is.

For instance, GitHub will automatically display the license it detected in this summary of the repository:

Note that you should also update the license field in the opam file.

[yforster]

Maybe we leave the commit as is, and put a commit on top that removes the CECILL file and the scripts subdir? That way the information is not lost because it strays part of the git tree, but it also stays around too prominently

[DmxLarchey]

@Zimmi48 thx for the opam tip ...

[DmxLarchey]

Ok so I squash merge this PR and then submit another one that removes the reference to the former CeCILL license and the scripts subdir.