nqp::split should allow for boxing type

[lizmat]

Or is should just return an nqp::list_s rather than an nqp::list with boxed strings.

nqp::split is documented to take 2 parameters: the delimiter and the string. It can actually take a third parameter, of which the usage is undocumented (and I've not been able to find a usage of in the NQP code base).

[MasterDuke17]

nqp::split is documented to take 2 parameters: the delimiter and the string. It can actually take a third parameter, of which the usage is undocumented (and I've not been able to find a usage of in the NQP code base).

If #714 is merged then nqp::split won't accept a third parameter anymore (in fact no ops will accept as a write destination, one more parameter than they're documented with).