Performance regression caused by Handlebars upgrade

A performance regression was introduced in Handlebars starting with Handlebars 4.3. This affects versions of Antora from 2.1.2 to 2.2.0. Due to this problem, building a large site can take as much as 5x as long. It's likely a bad interaction with Opal objects. This problem appears to have been fixed in Handlebars 4.7. So the solution is to simply upgrade. I can confirm that the performance returns to what we had before the 2.1.2 release.