Full PHP 7.4 support!

• Native property types (e54a7a5), link to RFC
• Arrow functions (c78f503), link to RFC
• Null coalesce assign operator ??= (9552811), link to RFC
• Array spread operator (a3bcc58), link to RFC
• Password hashing algorithm constant type is mixed (4c6c84c)
• Signatures about new PHP 7.4 functions (493d12d)
• Fixes deprecated call to ReflectionType::__toString() in PHP 7.4 (#2275), thanks @brunowowk!

Bugfixes

• Inferring private property types - do not propagate ErrorType/NeverType (60ab154)