An unpacked call carries a single Node\Arg regardless of its runtime
arity, so the arity check alone still accepted intval(...$args) as a
single-argument Native conversion and lowered the array itself:
intval(...['ff', 16]) // php::toInt(withBase) -> int(1)
strval(...['42']) // php::toString(single) -> "Array" + warning
A named argument has the same shape and need not be the value being
converted: intval(bogus: 1) must raise "Unknown named parameter", not
fold to a cast of 1.
Reject both in dispatchConversion() so the runtime determines the
expanded arity and the parameter names, matching what
dispatchFuncCall() already does for every other builtin.
Giandonn