Remove redundant is_dyn_thread_safe checks

[zetanumbers]

Refactor uses of FromDyn to reduce number of redundant is_dyn_thread_safe checks and add DynThreadSafe type to encourage users to avoid those in the future.

PR is split up into multiple commits for an easier review.

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[Zoxc]

The change to reduce the number of checks is good here.

I'm not sure about adding a separate type for it. The new type DynThreadSafe is equivalent to FromDyn<()> and the same change could be done by adding a FromDyn::try_from method. The naming of DynThreadSafe seem quite trait-like to me too. It seems like it could be more something like DynThreadSafetyEnabled.

[zetanumbers]

The naming of DynThreadSafe seem quite trait-like to me too.

Yes. That's because traits are predicates over types. Analogously DynThreadSafe is also a predicate but over a static variable. These are the same in dependent type systems.

[zetanumbers]

I'm not sure about adding a separate type for it. The new type DynThreadSafe is equivalent to FromDyn<()> and the same change could be done by adding a FromDyn::try_from method.

FromDyn is simply a newtype over a pair (T, DynThreadSafe) to implement Send and Sync. If those weren't rust traits which require a unique implementation then having a newtype wouldn't be necessary.