There are no OOTB solutions for this since the SharePoint Online cmdlets are very limited, and Export-Import doesn't exist today.
However, you can save your subsite as template
/_layouts/15/savetmpl.aspx and include content. When saved download the template from the solution gallery by clicking on the template name
Create a new site collection in the SharePoint admin center, and in the custom tab select template later:
When the site collection is created, browse to the new site and hit the solution link. Upload the template and activate it. Go back to your root site url, and select your custom template.
Now you have "moved" your subsite to a site collection of its own, but without PowerShell.
First, create a Site Collection on url https://foo.sharepoint.com/sites/bar with the same template as your subsite https://foo.sharepoint.com/bar.
Second, use Export-SPWeb
Export-SPWeb https://foo.sharepoint.com/bar -Path "d:\temp\bar-export.cmp"
Third, use Import-SPWeb
Import-SPWeb https://foo.sharepoint.com/sites/bar -Path "d:\temp\bar-export.cmp" -Force
Now, verify that the imported web is equal to your subsite, before you delete the subsite.