Prepare for 0.9.0 release

[nagisa]

cc @AlexanderSchuetz97 double-check that this works for you.

I ended up nuking a number of AsFilename and AsSymbolName impls still and made the remaining ones terser. We can easily add them back later on in a backwards-compatible manner if it turns out that the current impls don't serve some specific need for some reason or another.

[nagisa]

Unfortunately the supertrait sealing makes the implementers not visible in the documentation, which I think is crucial to have here. We can explore other options, although at the same time: are these suggestions showing up even when the trait isn't imported? There should rarely be a need for anybody to import the trait.

[nagisa]

Hm, maybe we could still use super trait and have the methods on it, but instead of having a generic impl<T: SealedTrait> Trait for T, we'd have implementations of Trait for each individual type too... Let me try.

[nagisa]

Also, why does this showing up in the documentation matter?
The user of libloading should never call these functions, they are something internal to the crate.

That's how you learn about which types you can use to pass in into Library::new or Library::get. One could argue that listing them manually somewhere would be good enough, but nothing really beats being able to click on AsSymbol and finding the following:

The internal method does not show up (in fact I especially hid it with doc(hidden) in the sealed method approach.) That said my idea of adjusting supertrait method does also work for doc generation so I'm happy to switch back to supertrait method.