Enable use of custom serde serializer on nbt arrays

[Schuwi]

Because of a little oversight on my part when implementing the ser::ixx_array functions they currently take a &'a T: IntoIterator as their array reference parameter, which is only ever used for a call of .into_iterator on the passed argument. Thus the trait bound can be simplified to T: IntoIterator which would allow ser::ixx_array to be called manually (i.e. from a custom with_serializer implementation) with an iterator as the array reference parameter to manipulate the array values before serialization as used in the correspondingly implemented serialize_custom_serializer_array test.
This PR also rewrites parts of the roundtrip_nested_array test to make sure that no unnecessary memory allocation occurs on serialization.

[Schuwi]

Okay, I ran another test with a real implementation of a custom serializer and everything seems to work fine

[atheriel]

Interesting test case. This looks good, thanks!