Reduce the visibility of types/methods throughout the crate

[muzarski]

A lot of defined types are pub for no reason. Many of them could probably be pub(crate) (or even pub(self)). Currently, the only items that need to be pub are ones that are either:

• a part of public C API
• used in the doctests

Maybe if we add Rust integration testing module in the future, there will be more items that need to be pub, but currently I don't see a reason not to decrease the visibility.

[wprzytula]

Done in #318.