* ssz: move ref support outside
Instead of allocating ref's inside SSZ, move it to separate helper:
* makes `ref` allocations explicit
* less magic inside SSZ
* `ref` in nim generally means reference whereas SSZ was loading as
value - if a type indeed used references it would get copies instead of
references to a single value on roundtrip which is unexpected
TODO: EF tests would benefit from some refactoring since they all do the
same thing practically..
Co-authored-by: Zahary Karadjov <zahary@gmail.com>
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