Varray example

[n-osborne]

This PR add the varray example. It is bassed on the not yet merged #177, please consider only the two last commits.

There were four challenges, of different importance:

• The specifications are in an ml file and Gospel does not read ml file. The proposed solution/workaround is to extract the content of the module signature we are interested in.
• Writing specifications with Gospel sequence type is not that easy (it is very possible that review reveals some off-by-one mistakes!)
• User have to write some dune machinery in order to test different Functor instantiations.
• The included module is a bit bigger than the one for the #177 example and need to be included in two different tests. We had to make it a library. The size of the included module is what triggered #181.

[n-osborne]

Waiting for varray.0.2 so that minimal version for varray can be added.

[n-osborne]

CI failure seems unrelated. I merge.