Generics signature help should include type parameter defaults

[jonas-schievink]

For example, in std::ops::ControlFlow::<$0, the signature help is rendered as enum ControlFlow<B, C>, which does not tell the user that C is optional and only B needs to be provided. Rendering the default type would fix that, and result in enum ControlFlow<B, C = ()>.

It's possible that a default type would be too long when fully rendered, so the length should probably be limited (the HirDisplay machinery can already do that).

[lowr]

I'm working on this. (I hope I'm not conflicting with anyone else...:sweat_smile:)

Should the limit on the rendered type length be configurable like rust-analyzer.inlayHints.maxLength, or can it be a hard-coded arbitrary value?