Prevent unreachable match arms

[Licenser]

Describe the problem you are trying to solve

when writing a match statement it can happen that match arms become unreachable. This is generally a mistake and should lead to an error or at least a warning.

match event of
  case 1 => "event is one"
  case 2 => "event is two"
  case 3 => "event is three"
  case 1 => "event is four" # oopsie typo!!!
...
end

or less obvious (we might not catch this!):

match event of
  case %{data ~= re|(badger )?[sS]not.*|} => "event is badger snot"
  case %{data ~= glob|snot*|} => "event is snot"
...
end

Describe the solution you'd like

test if any of the cases following a statement is a complete subset of the current case

Notes

We will not get 100% on this given how many extractors we have but it's still worth getting to 80%