NQP: Add defined(), exists(), undef.

[Austin_Hastings]

Testing using code like

   if $foo ...

works until your class doesn't have a get_boolean, or until you are trying to see "if $foo is set" rather than "if $foo is true".

Then you need defined, or some kind of ==undef or !=undef test.

[pmichaud@…]

Note that any functions such as defined(), undef(), etc. probably
ought to be part of a library (to be explicitly imported) rather
than built-in as defaults to NQP.

Pm

[jimmy]

Please use NQP-rx.