Unify experimental includes

[andreasbuhr]

This pull request cares about three issues:

1. The #include <coroutine> vs. #include <experimental/coroutine> problem. A header is introduced which checks which include is available.
2. The std:: vs. std::experimental:: namespace issue. Coroutine related things a defined into the cppcoro:: namespace.
3. The same for filesystem vs experimental/filesystem

This pull request is based on the pull request https://github.com/lewissbaker/cppcoro/pull/158 by @dutow. Thanks to @dutow for the great work. Pull request 158 addresses several issues at the same time. I extracted this pull request which cares only about one single issue, in order to have an easy to review pull request.

To help improve this pull request, please fork https://github.com/andreasbuhr/cppcoro/tree/unify_experimental_includes and create a pull request toward this branch.

[dutow]

@andreasbuhr Something isn't right here, the PR says that you extracted parts of my pull request here, but in truth, all my commits (and @mmha 's commit too, and a few from you) is included in this PR, along with some merge commits.

If this pull request is only the experimental include fix, shouldn't it just be a single commit, the cherry-pick of 0b201f8aac3787e0037d82a4c298f9c91db8c3f6?

[andreasbuhr]

Yes, you are right. I cherry-picked your whole pull request. I see three options:

1. Leave it like this, accepting clutter in the history.
2. Recreating this pull request as a cherry-pick of https://github.com/lewissbaker/cppcoro/commit/0b201f8aac3787e0037d82a4c298f9c91db8c3f6
3. Leave it like this, squashing it into one commit on merge. What would you prefer?

[dutow]

You can also fix this PR by (with your unify_experimantal_includes branch checked out locally):

git reset --hard master
git cherry-pick 0b201f8
git push --force-with-lease

[lewissbaker]

I see three options:

1. Leave it like this, accepting clutter in the history.
2. Recreating this pull request as a cherry-pick of 0b201f8
3. Leave it like this, squashing it into one commit on merge. What would you prefer?

I'll just squash the changes during the merge.

[andreasbuhr]

This pull request is now open for two month. I now stop maintaining this pull request. From now on, I will continue to maintain the master branch in https://github.com/andreasbuhr/cppcoro. There we have CMake support, a CI based on Github Actions, and support for the latest compilers.