* Add ci/common/submit_coverage.sh, used with Travis and AppVeyor
* use gcovr, with coverage.xml for better branch coverage reporting, and
easier processing of gcov files in general
* codecov: use flags again, with `uname -s` additionally
Ref: https://github.com/neovim/neovim/pull/10227#issuecomment-502923543
* remove now unused parsers.gcov config from codecov.yml
