3761: Append new match arms rather than replacing all of them r=matklad a=mattyhall
This means we now retain comments when filling in match arms. This fixes#3687. This is my first contribution so apologies if it needs a rethink! I think in particular the way I find the position to append to and remove_if_only_whitespace are a little hairy.
Co-authored-by: Matthew Hall <matthew@quickbeam.me.uk>