6.6.0: Performance, code coverage improvements

• Emit null-coalesce operator as $a ?? $a= expression instead of as
  $a= $a ?? expression, saving one assignment operation for non-null
  case. Applies to PHP 7.0, 7.1 and 7.2.
  (@thekid)
• Removed conditional checks for PHP 8.1 with native enum support, all
  releases and builds available on CI systems now contain it.
  (@thekid)
• Increased test coverage significantly (to more than 90%), especially
  for classes used by the compiler command line.
  (@thekid)