Support for PHP 7.4 new features

[ezzatron]

There are some new features in PHP 7.4 which may require new code for proper support. Namely:

• Weak references (Consider deeply rendering weak references. #245)
• Reference reflection (Possible issues with array reference detection in PHP 7.4 #244)
  • Probably most relevant in the exporter
  • Might also affect deep comparison
• Arrow functions
  • Need to check that they can be stubbed/spied with no issues
  • Might be useful for simplifying example code in the docs
• Typed properties
  • Need to check that they can be mocked with no issues
• Custom object serialization
  • Check that objects that use these methods can be mocked
  • Check that custom serialization methods can be stubbed
• Exceptions are now possible in __toString
  • Check that mocking doesn't interfere with this behavior

Relates to #246