846: WIP: Enable parsing of attributes inside a match block r=matklad a=vipentti
We allow invalid inner attributes to be parsed, e.g. inner attributes that are
not directly after the opening brace of the match block.
Instead we run validation on `MatchArmList` to allow better reporting of errors.
This fixes#845 and works towards #759
Co-authored-by: Ville Penttinen <villem.penttinen@gmail.com>