Mustache.php v3.0 will drop support for PHP <5.6.

• No more warnings or errors in PHP 8.x (thanks @reedy, @swissspidy, and everyone else who helped with this!).
• Finally get rid of the pseudo-namespaced classes (\Mustache_Engine -> \Mustache\Engine) like it's 2009.
• Preserve compatibility with non-namespaced class names (e.g. \Mustache_Engine) because let's not be too hasty.
• Remove autoloader implementation. Just use Composer.

This release should be a drop-in replacement for Mustache.php v2.x, provided you're using at least PHP 5.6. You are using at least PHP 5.6, right?

• In order to preserve the wide PHP version support range, there are some minor changes to interfaces, which you might need to handle if you extend Mustache (see c0453be5c09e7d988b396982e29218fcb25b7304).