Add a test to check mechanism type variants

[leamingrad]

This PR enhances p11_mechanism_type in a couple of ways:

• Move to using ppx_variants_conv to generating the list of all elements of the type. This will cause a compiler warning if new elements are added and omitted from the list
• Add a test that encodes all the variants as a string and then decodes them

[leamingrad]

I'm not a huge fan of ppx_variants_conv - in particular, I can't work out if there is a better way to generate the list of variants without duplicating the list.

[leamingrad]

is there by any chance a way to only derive this Variants.fold function ?

I haven't been able to find one. I guess the way to do it would be to extract the relevant pre-processor code from ppx_variants_conv and including it in the package. I'm not hugely keen on that as it doesn't seem like a great fit for this codebase.

I think the "correct" solution here would probably be to create a new ppx_* package that is a tightened up version of ppx_variants_conv that is more tuned to the needs of types that are basically just a load of nullary constructors with a a couple of special cases (or even just for all-nullary classes). Then we can swap that in here. Thats probably something for another time though.

[NathanReb]

Agreed, it's already a good improvement as it is!