Revert "Disable warnings by default"

This reverts commit be76b01a8758b7f5e6660fcf69990728a799bf5b.
Proper fix to disable warnings by default
Std: add List.uniq and List.sort_uniq
Move error cleanup from Error_report to Command

Filter duplicate errors which might arise from backtracking/recovery.
Add "exception _ with _" to 4.02 syntax, fix #255
type enclosing: more precise smallest enclosing