fix a few dead links
* Links in `Input` missed a refactor
* `Reflect::downcast` can't use the intra doc link format, as it's not a link to a trait function, but to a function implemented on `dyn Reflect`
noticed in https://github.com/bevyengine/bevy/pull/1781#discussion_r619777879
Fixes#1100
Implementors must make sure that `Reflect::any` and `Reflect::any_mut` both return the `self` reference passed in (both for logical correctness and downcast safety).
* Rename reflect 'hash' method to 'reflect_hash' to avoid colliding with std:#️⃣:Hash::hash to resolve#943.
* Rename partial_eq to reflect_partial_eq to avoid collisions with implementations of PartialEq on primitives.
