Show substitution where hovering over generic things

There are few things to note in the implementation:

First, this is a best-effort implementation. Mainly, type aliases may not be shown (due to their eager nature it's harder) and partial pathes (aka. hovering over `Struct` in `Struct::method`) are not supported at all.

Second, we only need to show substitutions in expression and pattern position, because in type position all generic arguments always have to be written explicitly.

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