serix: check registered interfaceObjects rules if the container is a slice of interfaces

[muXxer]

As mentioned in #658, serix doesn't check if the immutable features for an account output are valid.

This is because the AccountOutputImmFeatures are a slice of interfaces, and serix jumps into the encodeSlice code path, where it is never checked if the objects are registered for that interface.

[muXxer]

The problem was actually, that the Features[T] type ignored the T, so everything was a Feature instead of the specific features per output.