Implementation of some traits for their Deref counterparts.

[hdlj]

Traits concerned

• FstOp
• FstOp2
• TrMapper
• FstCache

Remove specific implementation for Arc.

By this way, this traits are now available on Rc and Arc for instance.

[kali]

while on the topic, do we have a compelling reason why determinize needs an Arc and not just an &Fst?

[Garvys]

yes I need to find a way to avoid that. The issue is that determinize (static version) shares most of the code of the lazy version. And in the lazy implementation, the Fst to be determinized is stored as an arc for different reasons:

• Avoid having to handle lifetime everywhere
• If we have an &Fst, then the input fst need to stay on scope as long as the DeterminizedFst (lazy version) is on scope. By using Arc, it is done automatically.
  This is the case for other algorithms that share a big part of their code between the static version and the lazy version like the composition.