Remove unnecessary usage of GADTs

Co-authored-by: Mateusz Galazyn <[email protected]>

Author: palas

docs/ADR-10-Callstacks-for-API-errors.md

@@ -65,8 +65,7 @@ class Error e where
   getErrorCallStack :: e -> CallStack
 ```
 
-We need a `Content` wrapper as a `GADT`, to prevent the type of the `ErrorContent` from propagating to the type of `Error`, while ensuring that it is indeed an `ErrorContent`.
-
+We need `e` in `Content` wrapper to be an existential type, to prevent the type of the `ErrorContent` from propagating to the type of `Error`, while ensuring that it is indeed an `ErrorContent`.
 If we do this, we could define two generic functions for every `Error e`.
 
 One to print the original error, without a stack-trace:
@@ -172,7 +171,7 @@ But, other than that, it comes down to adding a space for the stack-trace in the
 
 At least, we can have a reusable wrapper for errors (that must now implement the `ErrorContent` class).
 
-So we can define an `ErrorWithStack` data type using a `GADT` as follows:
+So we can define an `ErrorWithStack` data type:
 
 ```haskell
 data ErrorWithStack e where