Use features of PHP 7.2

[Alexey Borzov]

• Change read-only arrays to constants (e.g. Lexer::$keywords)
• Some methods that accept arrays can accept (...$stuff) (e.g. new QualifiedName('foo', 'bar') rather than new QualifiedName(['foo', 'bar']))
• Use traits (e.g. several leaf Nodes share the same setParentNode() implementation)
• Binary numbers for Token constants

And the obvious: strict_types, scalar type hints, types for return values