faust-types pulls in jack dependency

[obsoleszenz]

Currently faust_types has a default jack feature which for me feels weird. faust-types should not pull in the jack dependency.

I'm wondering if we actually need the jack dependency at all in faust-types. The libm dependency I could imagine we need, I think I remember that the rust std library didn't offer a couple of math methods that faust needs?

@amomentunfolding You made the import of libm and jack for faust-types optional in 63c0421 . Maybe you have a clearer idea on this?

Didn't play around with this yet, so this is mainly an issue to document that we can make things cleaner here.

[crop2000]

i think it makes sense to rename the feature flag to libm and remove jack which is not used at all.