6.7.0: Native callable syntax

• Changed emitter to omit extra newlines between members, making line
  numbers more consistent with the original code.
  (@thekid)
• Merged PR #114: Implements first-class callable syntax: strlen(...)
  now returns a closure which if invoked with a string argument, returns
  its length. Includes support for static and instance methods as well as
  indirect references like $closure(...) and self::{$expression}(...),
  see https://wiki.php.net/rfc/first_class_callable_syntax
  (@thekid)

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)

6.5.0: Declare directives

• Merged PR #111: Add support for directives using declare - @thekid

6.4.0: Never return type

• Merged PR #110: Rewrite never return type to void in PHP < 8.1, adding
  support for this PHP 8.1 feature. XP Framework reflection supports this
  as of its 10.10.0 release.
  (@thekid)

6.3.2: Anonymous child classes

• Allowed new class() extends self inside class declarations - @thekid

6.3.1: Cloning fix

• Fix being able to clone enum lookalikes - @thekid
• Fix clone operator - @thekid

6.3.0: PHP enum support

• Merged PR #106: Compile PHP enums to PHP 7/8 lookalikes, PHP 8.1 native.
  (@thekid)

6.2.0: Arbitrary expressions as initializers

• Merged PR #104: Support arbitrary expressions in property initializations
  and parameter defaults
  (@thekid)

6.1.1: Nullable types fixes

• Fixed issue #102: Call to a member function children()... - @thekid
• Fixed issue #102: PHP 8.1 compatibility - @thekid
• Fixed issue #103: Nullable types - @thekid

6.1.0: Show languages and emitters

• Included languages and emitters in xp compile output - @thekid