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)