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.

crates/ide/src/goto_declaration.rs

@@ -36,7 +36,7 @@ pub(crate) fn goto_declaration(
             let def = match_ast! {
                 match parent {
                     ast::NameRef(name_ref) => match NameRefClass::classify(&sema, &name_ref)? {
-                        NameRefClass::Definition(it) => Some(it),
+                        NameRefClass::Definition(it, _) => Some(it),
                         NameRefClass::FieldShorthand { field_ref, .. } =>
                             return field_ref.try_to_nav(db),
                         NameRefClass::ExternCrateShorthand { decl, .. } =>