Better error messages for the monomorphism restriction

Today [Competitive_Ad2539 on Reddit asks why type inference fails for this code](https://reddit.com/r/haskellquestions/comments/sp9u4a/why_did_the_type_inference_failed_here/):

```haskell
showLine = (++ "\n") . concat
```

The error message is (with GHC 9.2.1 and 9.0.2):

```
T.hs:1:24: error:
* Ambiguous type variable `t0' arising from a use of `concat'
prevents the constraint `(Foldable t0)' from being solved.
Relevant bindings include
showLine :: t0 [Char] -> [Char] (bound at T.hs:1:1)
Probable fix: use a type annotation to specify what `t0' should be.
These potential instances exist:
instance Foldable (Either a) -- Defined in `Data.Foldable'
instance Foldable Maybe -- Defined in `Data.Foldable'
instance Foldable ((,) a) -- Defined in `Data.Foldable'
...plus two others
...plus 26 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
* In the second argument of `(.)', namely `concat'
In the expression: (++ "\n") . concat
In an equation for `showLine': showLine = (++ "\n") . concat
|
1 | showLine = (++ "\n") . concat
| ^^^^^^
```

Experts might see at a glance that this is probably caused by the monomorphism restriction, but that is not at all obvious from this error message.

I would propose adding a note in the error message that the monomorphism restriction prevents it from inferring the most general type.