git subrepo pull packages/PEGTL

subrepo:
  subdir:   "packages/PEGTL"
  merged:   "3d4fee283"
upstream:
  origin:   "git@github.com:taocpp/PEGTL.git"
  branch:   "master"
  commit:   "3d4fee283"
git-subrepo:
  version:  "0.4.1"
  origin:   "git@github.com:ingydotnet/git-subrepo.git"
  commit:   "a04d8c2"

Due to changes in PEGTL, we now use our own ParseError class.
Will have to take benefits from the new unwind feature to report errors