Overlapping instances errors can be improved

This issue is inspired by this question on discourse: https://discourse.haskell.org/t/issue-with-overlapping-instances/11467?u=jaror

Here are some short notes on how the message could be improved:

* If kind variables play an essential role in rejecting overlap, then they should be printed in the message!
* Ideally, the message would clearly indicate which parts of the instance heads cause the conflicting specificity. For example here's how I'd improve this specific message manually:
```
An overlapping instance can only be chosen when it is strictly more specific.
The first instance that follows overlaps the second, but neither is more specific than the other:
instance [overlappable] (...) Embeds @{k} (t n) (t m)
instance [overlapping] (...) Embeds @{*} n (t m)
^ ^
| +-- 't n' is more specific than 'n'
+----- 'k' is less specific than '*'
```