Due to #20400 the corresponding TrustedLen impls need a helper trait
instead of directly adding `Item = &[T;N]` bounds.
Since TrustedLen is a public trait this in turn means
the helper trait needs to be public. Since it's just a workaround
for a compiler deficit it's marked hidden, unstable and unsafe.
This only works if arrays are passed directly instead of array iterators
because we need to be sure that they have not been advanced before
Flatten does its size calculation.
