ssz assert to fix list decoding

[turuslan]

sszpp decoding breaks if type is not marked with ssz::ssz_variable_size_container.
Add assert to check fixed and variable size consistency.

#define SSZ_CONT(...)
+  static_assert(variable_size::value or ssz::tuple_all_fixed_size<decltype(t)>::value);

https://github.com/leanEthereum/leanSpec/blob/f57fca81ca3a4cf87ee39ab993694f297b7e9a3b/src/lean_spec/types/container.py#L45-L89